* Filesystem unable to recover from ENOSPC @ 2014-04-10 20:00 Chip Turner 2014-04-10 20:34 ` Hugo Mills 0 siblings, 1 reply; 6+ messages in thread From: Chip Turner @ 2014-04-10 20:00 UTC (permalink / raw) To: linux-btrfs I have a filesystem that I can't seem to resolve ENOSPC issues. No write operation can succeed; I've tried the wiki's suggestions (balancing, which fails because of ENOSPC, mounting with nodatacow, clear_cache, nospace_cache, enospc_debug, truncating files, deleting files, briefly microwaving the drive, etc). btrfs show: Label: none uuid: 04283a32-b388-480b-9949-686675fad7df Total devices 1 FS bytes used 135.58GiB devid 1 size 238.22GiB used 238.22GiB path /dev/sdb2 btrfs fi df: Data, single: total=234.21GiB, used=131.82GiB System, single: total=4.00MiB, used=48.00KiB Metadata, single: total=4.01GiB, used=3.76GiB So, the filesystem is pretty much unusable, and I can find no way to resuscitate it. I ended up in this state by creating a snapshot of the root of the fs into a read/write subvolume, which I wanted to become my new root, then began deleting entries in the filesystem itself outside of the new snapshot. So nothing particularly weird or crazy. The only oddness is the file count -- I have a *lot* of hardlinked files (this is an rsnapshot volume, so it has a large number of files and many of them are hard linked). It seems like the normal solution is btrfs balance, but that fails. defragment also fails. Kernel is 3.13. Is there anything else I can or should do, or just wipe it and recreate with perhaps better initial defaults? If this kind of thing is unavoidable, how might I have anticipated it and prevented it? Fortunately this was a migration effort and so my original data is safe inside of ext4 (ew). -- Chip Turner - cturner@pattern.net ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Filesystem unable to recover from ENOSPC 2014-04-10 20:00 Filesystem unable to recover from ENOSPC Chip Turner @ 2014-04-10 20:34 ` Hugo Mills 2014-04-10 22:40 ` Chip Turner 0 siblings, 1 reply; 6+ messages in thread From: Hugo Mills @ 2014-04-10 20:34 UTC (permalink / raw) To: Chip Turner; +Cc: linux-btrfs [-- Attachment #1: Type: text/plain, Size: 2662 bytes --] On Thu, Apr 10, 2014 at 01:00:35PM -0700, Chip Turner wrote: > I have a filesystem that I can't seem to resolve ENOSPC issues. No > write operation can succeed; I've tried the wiki's suggestions > (balancing, which fails because of ENOSPC, mounting with nodatacow, > clear_cache, nospace_cache, enospc_debug, truncating files, deleting > files, briefly microwaving the drive, etc). > > btrfs show: > Label: none uuid: 04283a32-b388-480b-9949-686675fad7df > Total devices 1 FS bytes used 135.58GiB > devid 1 size 238.22GiB used 238.22GiB path /dev/sdb2 > > btrfs fi df: > Data, single: total=234.21GiB, used=131.82GiB > System, single: total=4.00MiB, used=48.00KiB > Metadata, single: total=4.01GiB, used=3.76GiB I'm surprised it's managed to use that much of the metadata allocation. The FS usually hits this problem with a much smaller used-to-total ratio. > So, the filesystem is pretty much unusable, and I can find no way to > resuscitate it. I ended up in this state by creating a snapshot of > the root of the fs into a read/write subvolume, which I wanted to > become my new root, then began deleting entries in the filesystem > itself outside of the new snapshot. So nothing particularly weird or > crazy. The only oddness is the file count -- I have a *lot* of > hardlinked files (this is an rsnapshot volume, so it has a large > number of files and many of them are hard linked). > > It seems like the normal solution is btrfs balance, but that fails. > defragment also fails. Kernel is 3.13. > > Is there anything else I can or should do, or just wipe it and > recreate with perhaps better initial defaults? If this kind of thing > is unavoidable, how might I have anticipated it and prevented it? > Fortunately this was a migration effort and so my original data is > safe inside of ext4 (ew). One thing you could do is btrfs dev add a small new device to the filesystem (say, a USB stick, or a 4 GiB loopback file mounted over NBD or something). Then run the filtered balance. Then btrfs dev del the spare device. The fact that this FS has ended up in this state should be considered a bug. It used to be quite common, then josef fixed a load of problems, and it's been rare for about a year. Only recently we've been seeing more of this kind of problem, and I think there's been a bit of a regression somewhere. Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- "Can I offer you anything? Tea? Seedcake? --- Glass of Amontillado?" [-- Attachment #2: Digital signature --] [-- Type: application/pgp-signature, Size: 811 bytes --] ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Filesystem unable to recover from ENOSPC 2014-04-10 20:34 ` Hugo Mills @ 2014-04-10 22:40 ` Chip Turner 2014-04-11 2:09 ` Duncan 0 siblings, 1 reply; 6+ messages in thread From: Chip Turner @ 2014-04-10 22:40 UTC (permalink / raw) To: Hugo Mills, Chip Turner, linux-btrfs On Thu, Apr 10, 2014 at 1:34 PM, Hugo Mills <hugo@carfax.org.uk> wrote: > On Thu, Apr 10, 2014 at 01:00:35PM -0700, Chip Turner wrote: >> btrfs show: >> Label: none uuid: 04283a32-b388-480b-9949-686675fad7df >> Total devices 1 FS bytes used 135.58GiB >> devid 1 size 238.22GiB used 238.22GiB path /dev/sdb2 >> >> btrfs fi df: >> Data, single: total=234.21GiB, used=131.82GiB >> System, single: total=4.00MiB, used=48.00KiB >> Metadata, single: total=4.01GiB, used=3.76GiB > > I'm surprised it's managed to use that much of the metadata > allocation. The FS usually hits this problem with a much smaller > used-to-total ratio. It perhaps was related to how I build it -- basically I dumped many files (around 8.5 million, 95% of which are hardlinks rather than actual files) onto it at once, created a snapshot, and then began deleting files. The btrfs filesystem they were coming from was fine with the files and its own original load, but something about the snapshotting and deleting seemed to push it over the edge. The issue happened over a couple of days so I don't recall exactly, but I may also have run out of space and had to delete a lot of files (basically, I left off the '-H' to rsync, so, when I copied to the system, it made copies rather than hard links). This occurred before the original full copy and hence before the snapshot and deletion. > One thing you could do is btrfs dev add a small new device to the > filesystem (say, a USB stick, or a 4 GiB loopback file mounted over > NBD or something). Then run the filtered balance. Then btrfs dev del > the spare device. Ah, this worked great. It fixed it in about ten seconds. I'm curious about the space report; why doesn't Data+System+Metadata add up to the total space used on the device? Was the fs stuck in a state where it couldn't clean up because it couldn't write more metadata (and hence adding a few gb allowed it to balance)? After the balance, the used space dropped to around 150GB, roughly what I'd expect. > The fact that this FS has ended up in this state should be > considered a bug. It used to be quite common, then josef fixed a load > of problems, and it's been rare for about a year. Only recently we've > been seeing more of this kind of problem, and I think there's been a > bit of a regression somewhere. Is there any other data I can provide to help fix this? I can see if it's reproducible (I'm migrating disks so I can just keep doing it and see if it happens again). Chip -- Chip Turner - cturner@pattern.net ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Filesystem unable to recover from ENOSPC 2014-04-10 22:40 ` Chip Turner @ 2014-04-11 2:09 ` Duncan 2014-04-11 6:10 ` Chip Turner 0 siblings, 1 reply; 6+ messages in thread From: Duncan @ 2014-04-11 2:09 UTC (permalink / raw) To: linux-btrfs Chip Turner posted on Thu, 10 Apr 2014 15:40:22 -0700 as excerpted: > On Thu, Apr 10, 2014 at 1:34 PM, Hugo Mills <hugo@carfax.org.uk> wrote: >> On Thu, Apr 10, 2014 at 01:00:35PM -0700, Chip Turner wrote: >>> btrfs show: >>> Label: none uuid: 04283a32-b388-480b-9949-686675fad7df >>> Total devices 1 FS bytes used 135.58GiB >>> devid 1 size 238.22GiB used 238.22GiB path /dev/sdb2 >>> >>> btrfs fi df: >>> Data, single: total=234.21GiB, used=131.82GiB >>> System, single: total=4.00MiB, used=48.00KiB >>> Metadata, single: total=4.01GiB, used=3.76GiB [Tried all the usual tricks, didn't work.] >> One thing you could do is btrfs dev add a small new device to the >> filesystem (say, a USB stick, or a 4 GiB loopback file mounted over NBD >> or something). Then run the filtered balance. Then btrfs dev del the >> spare device. > > Ah, this worked great. It fixed it in about ten seconds. > > I'm curious about the space report; why doesn't Data+System+Metadata add > up to the total space used on the device? Actually, it does... *IF* you know how to read it. Unfortunately that's a *BIG* *IF*, because btrfs show very confusingly reports two very different numbers using very similar wording, without making *AT* *ALL* clear what it's actually reporting. Try this: Add up the df totals (which is the space allocated for each category type). 234.21 gig, 4.01 gig, 4 meg. 238 gig and change, correct? Look at the show output. What number does that look like there? Now do the same with the df used (which is the space used out of that allocated). 131.82 gig, 3.76 gig, (insubstantial). 135 gig and change. What number from btrfs show does /that/ look like? Here's what's happening and how to read those numbers. Btrfs uses space in two stages. First it on-demand allocates chunks dedicated to the usage type. Data chunks are 1 GiB in size. Metadata chunks are 256 MiB in size, a quarter the size of a data chunk, altho by default on a single device they are allocated in pairs, dup mode, so 512 MiB at a time (half a data-chunk), tho I see your metadata is single mode so it's still only 256 MiB at a time. The space used by these ALLOCATED chunks appears as the totals in btrfs filesystem df and as used in btrfs filesystem show for individual devices, but the show total line comes from somewhere else *ENTIRELY*, which is why the reported individual device used number sum up to (if there's more than one device, the individual device numbers can be added together, if it's just one, that's it) so much larger than the number reported by show as total used. That metadata-single, BTW, probably explains Hugo's observation, that you were able to use more of your metadata than most, because you're running single metadata mode instead of the more usual dup. (Either you set it up that way, or mkfs.btrfs detected SSD, in which case it defaults to single metadata for a single device filesystem.) So you were able to get closer to full metadata usage. (Btrfs reserves some metadata, typically about a chunk, which means about two chunks in dup mode, for its own usage. That's never usable so it always looks like you have a bit more free metadata space than you actually do. But as long as there's unallocated free space to allocate additional metadata blocks from, it doesn't matter. Only when all space is allocated does it matter, since then it still looks like you have free metadata space to use when you don't.) Anyway, once btrfs has a chunk of the appropriate type, it fills it up. When necessary, it'll try to allocate another chunk. The actual usage of the already allocated chunks appears in btrfs filesystem df as used, with the total of all types for all devices also appearing in btrfs filesystem show on the total used line. So data+metadata+system allocated as reported by df, adds up to the totals reported as used by show for all the individual devices, added together. And data+metadata+system actually used (out of the allocated) as reported by df, adds up to the total reported by show as used, on the total used line. But they are two very different numbers, one total chunks allocated, the other the total used OF those allocated chunks. Makes sense *IF* *YOU* *KNOW* *HOW* *TO* *READ* *IT*, but otherwise, it's *ENTIRELY* misleading and confusing! There has already been discussion and proposed patches for adding more detail to df and show, with the wording changed up as well, and I sort of expected to see that in btrfs-progs v3.14 when it came out altho I'm running it now and don't see a change, but FWIW, from the posted examples at least, I couldn't quite figure out the proposed new output either, so it might not be that much better than what we have. Which might or might not have anything to do with it not appearing in v3.14 as I expected. Meanwhile, now that I actually know how to read the current output, it does provide the needed information, even if the output /is/ rather confusing to newbies. Back to btrfs behavior and how it leads to nospc errors, however... When btrfs deletes files, it frees space in the corresponding chunks, but since individual files normally use a lot more data space than metadata, data chunks get emptied faster than the corresponding metadata chunks. But here's the problem. Btrfs can automatically free space back to the allocated chunks as files get deleted, but it does *NOT* (yet) know how to automatically deallocate those now empty or mostly empty chunks, returning them to the unallocated pool so they can be reused as another chunk-type if necessary. So btrfs uses space up in two stages, but can only automatically return unused space back in one stage, not the other. Currently, to deallocate and free those unused blocks, you must run balance (which is where the filtered balance -dusage=20 or whatever, to balance, in that case only data chunks with 20% usage or less), which rewrites those blocks and consolidates any remaining usage as it goes, freeing up the blocks it empties back to the unallocated pool. At some point the devs plan to automate the process, probably by automatically triggering a balance start -dusage=5 or balance start -musage=5, or whatever, as necessary. But that hasn't happened yet. Which is why admins must currently keep an eye on things and run that balance manually (or hack up some sort of script to do it automatically, themselves) when necessary. > Was the fs stuck in a state > where it couldn't clean up because it couldn't write more metadata (and > hence adding a few gb allowed it to balance)? Basically, yes. As you can see from the individual device line in the above show output, 238.22 gig used (that is, chunks allocated), of 238.22 filesystem size. There's no room left to allocate additional chunks, not even one in ordered to rewrite the remaining data from some of those mostly empty data chunks, in ordered to return them to the unallocated pool. With a bit of luck, you would have had at least one entirely empty data chunk, in which case a balance start -dusage=0 would have freed it (since it was entirely empty and thus there was nothing to rewrite to a new chunk), thus giving you enough space to actually allocate a new chunk, to write into and free more of them. But if you tried a balance start -dusage=0 and it couldn't find even one entirely empty data chunk to free, as apparently you did, then you had a problem, since all available space was already allocated. Temporarily adding another device gave it enough room to allocate a few new chunks, such that balance then had enough space to rewrite a few of the mostly empty chunks, thereby freeing enough space so you could then btrfs device delete the new device, rewriting those new chunks back to the newly deallocated space on the original device. > After the balance, the > used space dropped to around 150GB, roughly what I'd expect. =:^) -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Filesystem unable to recover from ENOSPC 2014-04-11 2:09 ` Duncan @ 2014-04-11 6:10 ` Chip Turner 2014-04-11 9:28 ` Liu Bo 0 siblings, 1 reply; 6+ messages in thread From: Chip Turner @ 2014-04-11 6:10 UTC (permalink / raw) To: linux-btrfs Thank you for the extremely detailed and helpful reply. I now understand what was happening. To me, when I read "total=" I guess I thought that was capacity rather than allocated (but now "holey") chunks. I agree that perhaps adjusting the phrasing of the output of df and show would be helpful in making this clearer (or perhaps some adjustment to the wiki; maybe I will do that). I assume metadata is where filesystem information (directory entries, inodes, etc) are stored and not being able to delete was because there wasn't room in the metadata chunks, and no room to allocate more metadata chunks? Or are those also data chunks and it was purely about data chunks? Does defrag do similar reallocation to balance such that it would help combat this behavior? Thanks again, Chip On Thu, Apr 10, 2014 at 7:09 PM, Duncan <1i5t5.duncan@cox.net> wrote: > Chip Turner posted on Thu, 10 Apr 2014 15:40:22 -0700 as excerpted: > >> On Thu, Apr 10, 2014 at 1:34 PM, Hugo Mills <hugo@carfax.org.uk> wrote: >>> On Thu, Apr 10, 2014 at 01:00:35PM -0700, Chip Turner wrote: >>>> btrfs show: >>>> Label: none uuid: 04283a32-b388-480b-9949-686675fad7df >>>> Total devices 1 FS bytes used 135.58GiB >>>> devid 1 size 238.22GiB used 238.22GiB path /dev/sdb2 >>>> >>>> btrfs fi df: >>>> Data, single: total=234.21GiB, used=131.82GiB >>>> System, single: total=4.00MiB, used=48.00KiB >>>> Metadata, single: total=4.01GiB, used=3.76GiB > > [Tried all the usual tricks, didn't work.] > >>> One thing you could do is btrfs dev add a small new device to the >>> filesystem (say, a USB stick, or a 4 GiB loopback file mounted over NBD >>> or something). Then run the filtered balance. Then btrfs dev del the >>> spare device. >> >> Ah, this worked great. It fixed it in about ten seconds. >> >> I'm curious about the space report; why doesn't Data+System+Metadata add >> up to the total space used on the device? > > Actually, it does... *IF* you know how to read it. Unfortunately that's > a *BIG* *IF*, because btrfs show very confusingly reports two very > different numbers using very similar wording, without making *AT* *ALL* > clear what it's actually reporting. > > Try this: > > Add up the df totals (which is the space allocated for each category > type). 234.21 gig, 4.01 gig, 4 meg. 238 gig and change, correct? Look > at the show output. What number does that look like there? > > Now do the same with the df used (which is the space used out of that > allocated). 131.82 gig, 3.76 gig, (insubstantial). 135 gig and change. > What number from btrfs show does /that/ look like? > > Here's what's happening and how to read those numbers. Btrfs uses space > in two stages. > > First it on-demand allocates chunks dedicated to the usage type. Data > chunks are 1 GiB in size. Metadata chunks are 256 MiB in size, a quarter > the size of a data chunk, altho by default on a single device they are > allocated in pairs, dup mode, so 512 MiB at a time (half a data-chunk), > tho I see your metadata is single mode so it's still only 256 MiB at a > time. > > The space used by these ALLOCATED chunks appears as the totals in btrfs > filesystem df and as used in btrfs filesystem show for individual > devices, but the show total line comes from somewhere else *ENTIRELY*, > which is why the reported individual device used number sum up to (if > there's more than one device, the individual device numbers can be added > together, if it's just one, that's it) so much larger than the number > reported by show as total used. > > That metadata-single, BTW, probably explains Hugo's observation, that you > were able to use more of your metadata than most, because you're running > single metadata mode instead of the more usual dup. (Either you set it > up that way, or mkfs.btrfs detected SSD, in which case it defaults to > single metadata for a single device filesystem.) So you were able to get > closer to full metadata usage. (Btrfs reserves some metadata, typically > about a chunk, which means about two chunks in dup mode, for its own > usage. That's never usable so it always looks like you have a bit more > free metadata space than you actually do. But as long as there's > unallocated free space to allocate additional metadata blocks from, it > doesn't matter. Only when all space is allocated does it matter, since > then it still looks like you have free metadata space to use when you > don't.) > > Anyway, once btrfs has a chunk of the appropriate type, it fills it up. > When necessary, it'll try to allocate another chunk. > > The actual usage of the already allocated chunks appears in btrfs > filesystem df as used, with the total of all types for all devices also > appearing in btrfs filesystem show on the total used line. > > So data+metadata+system allocated as reported by df, adds up to the > totals reported as used by show for all the individual devices, added > together. > > And data+metadata+system actually used (out of the allocated) as reported > by df, adds up to the total reported by show as used, on the total used > line. > > But they are two very different numbers, one total chunks allocated, the > other the total used OF those allocated chunks. Makes sense *IF* *YOU* > *KNOW* *HOW* *TO* *READ* *IT*, but otherwise, it's *ENTIRELY* misleading > and confusing! > > There has already been discussion and proposed patches for adding more > detail to df and show, with the wording changed up as well, and I sort of > expected to see that in btrfs-progs v3.14 when it came out altho I'm > running it now and don't see a change, but FWIW, from the posted examples > at least, I couldn't quite figure out the proposed new output either, so > it might not be that much better than what we have. Which might or might > not have anything to do with it not appearing in v3.14 as I expected. > Meanwhile, now that I actually know how to read the current output, it > does provide the needed information, even if the output /is/ rather > confusing to newbies. > > Back to btrfs behavior and how it leads to nospc errors, however... > > When btrfs deletes files, it frees space in the corresponding chunks, but > since individual files normally use a lot more data space than metadata, > data chunks get emptied faster than the corresponding metadata chunks. > > But here's the problem. Btrfs can automatically free space back to the > allocated chunks as files get deleted, but it does *NOT* (yet) know how > to automatically deallocate those now empty or mostly empty chunks, > returning them to the unallocated pool so they can be reused as another > chunk-type if necessary. > > So btrfs uses space up in two stages, but can only automatically return > unused space back in one stage, not the other. Currently, to deallocate > and free those unused blocks, you must run balance (which is where the > filtered balance -dusage=20 or whatever, to balance, in that case only > data chunks with 20% usage or less), which rewrites those blocks and > consolidates any remaining usage as it goes, freeing up the blocks it > empties back to the unallocated pool. > > At some point the devs plan to automate the process, probably by > automatically triggering a balance start -dusage=5 or balance start > -musage=5, or whatever, as necessary. But that hasn't happened yet. > Which is why admins must currently keep an eye on things and run that > balance manually (or hack up some sort of script to do it automatically, > themselves) when necessary. > >> Was the fs stuck in a state >> where it couldn't clean up because it couldn't write more metadata (and >> hence adding a few gb allowed it to balance)? > > Basically, yes. As you can see from the individual device line in the > above show output, 238.22 gig used (that is, chunks allocated), of 238.22 > filesystem size. There's no room left to allocate additional chunks, not > even one in ordered to rewrite the remaining data from some of those > mostly empty data chunks, in ordered to return them to the unallocated > pool. > > With a bit of luck, you would have had at least one entirely empty data > chunk, in which case a balance start -dusage=0 would have freed it (since > it was entirely empty and thus there was nothing to rewrite to a new > chunk), thus giving you enough space to actually allocate a new chunk, to > write into and free more of them. But if you tried a balance start > -dusage=0 and it couldn't find even one entirely empty data chunk to > free, as apparently you did, then you had a problem, since all available > space was already allocated. > > Temporarily adding another device gave it enough room to allocate a few > new chunks, such that balance then had enough space to rewrite a few of > the mostly empty chunks, thereby freeing enough space so you could then > btrfs device delete the new device, rewriting those new chunks back to > the newly deallocated space on the original device. > >> After the balance, the >> used space dropped to around 150GB, roughly what I'd expect. > > =:^) > > > -- > Duncan - List replies preferred. No HTML msgs. > "Every nonfree program has a lord, a master -- > and if you use the program, he is your master." Richard Stallman > > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Chip Turner - cturner@pattern.net ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Filesystem unable to recover from ENOSPC 2014-04-11 6:10 ` Chip Turner @ 2014-04-11 9:28 ` Liu Bo 0 siblings, 0 replies; 6+ messages in thread From: Liu Bo @ 2014-04-11 9:28 UTC (permalink / raw) To: Chip Turner; +Cc: linux-btrfs On Thu, Apr 10, 2014 at 11:10:33PM -0700, Chip Turner wrote: > Thank you for the extremely detailed and helpful reply. I now > understand what was happening. To me, when I read "total=" I guess I > thought that was capacity rather than allocated (but now "holey") > chunks. I agree that perhaps adjusting the phrasing of the output of > df and show would be helpful in making this clearer (or perhaps some > adjustment to the wiki; maybe I will do that). > > I assume metadata is where filesystem information (directory entries, > inodes, etc) are stored and not being able to delete was because there > wasn't room in the metadata chunks, and no room to allocate more > metadata chunks? Or are those also data chunks and it was purely > about data chunks? Yes and no. Deleting metadata also needs metadata, and metadata chunk and data chunk are different ones. > > Does defrag do similar reallocation to balance such that it would help > combat this behavior? I'm afraid it won't, since you have that many hardlinks, defrag may not help on reducing metadata. thanks, -liubo ^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2014-04-11 9:28 UTC | newest] Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2014-04-10 20:00 Filesystem unable to recover from ENOSPC Chip Turner 2014-04-10 20:34 ` Hugo Mills 2014-04-10 22:40 ` Chip Turner 2014-04-11 2:09 ` Duncan 2014-04-11 6:10 ` Chip Turner 2014-04-11 9:28 ` Liu Bo
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