Re: Feature requests: online backup - defrag - change RAID level

[General Zed <general-zed@zedlx.com>]

Quoting "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:

> On 2019-09-12 18:21, General Zed wrote:
>>
>> Quoting "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:
>>
>>> On 2019-09-12 15:18, webmaster@zedlx.com wrote:
>>>>
>>>> Quoting "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:
>>>>
>>>>> On 2019-09-11 17:37, webmaster@zedlx.com wrote:
>>>>>>
>>>>>> Quoting "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:
>>>>>>
>>>>>>> On 2019-09-11 13:20, webmaster@zedlx.com wrote:
>>>>>>>>
>>>>>>>> Quoting "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:
>>>>>>>>
>>>>>>>>> On 2019-09-10 19:32, webmaster@zedlx.com wrote:
>>>>>>>>>>
>>>>>>>>>> Quoting "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:
>>>>>>>>>>
>>>>>>
>>>>>>>>> Given this, defrag isn't willfully unsharing anything, it's  
>>>>>>>>> just a side-effect of how it works (since it's rewriting the  
>>>>>>>>> block layout of the file in-place).
>>>>>>>>
>>>>>>>> The current defrag has to unshare because, as you said,  
>>>>>>>> because it is unaware of the full reflink structure. If it  
>>>>>>>> doesn't know about all reflinks, it has to unshare, there is  
>>>>>>>> no way around that.
>>>>>>>>
>>>>>>>>> Now factor in that _any_ write will result in unsharing the  
>>>>>>>>> region being written to, rounded to the nearest full  
>>>>>>>>> filesystem block in both directions (this is mandatory, it's  
>>>>>>>>> a side effect of the copy-on-write nature of BTRFS, and is  
>>>>>>>>> why files that experience heavy internal rewrites get  
>>>>>>>>> fragmented very heavily and very quickly on BTRFS).
>>>>>>>>
>>>>>>>> You mean: when defrag performs a write, the new data is  
>>>>>>>> unshared because every write is unshared? Really?
>>>>>>>>
>>>>>>>> Consider there is an extent E55 shared by two files A and B.  
>>>>>>>> The defrag has to move E55 to another location. In order to  
>>>>>>>> do that, defrag creates a new extent E70. It makes it belong  
>>>>>>>> to file A by changing the reflink of extent E55 in file A to  
>>>>>>>> point to E70.
>>>>>>>>
>>>>>>>> Now, to retain the original sharing structure, the defrag has  
>>>>>>>> to change the reflink of extent E55 in file B to point to  
>>>>>>>> E70. You are telling me this is not possible? Bullshit!
>>>>>>>>
>>>>>>>> Please explain to me how this 'defrag has to unshare' story  
>>>>>>>> of yours isn't an intentional attempt to mislead me.
>>>>>>
>>>>>>> As mentioned in the previous email, we actually did have a  
>>>>>>> (mostly) working reflink-aware defrag a few years back.  It  
>>>>>>> got removed because it had serious performance issues.  Note  
>>>>>>> that we're not talking a few seconds of extra time to defrag a  
>>>>>>> full tree here, we're talking double-digit _minutes_ of extra  
>>>>>>> time to defrag a moderate sized (low triple digit GB)  
>>>>>>> subvolume with dozens of snapshots, _if you were lucky_ (if  
>>>>>>> you weren't, you would be looking at potentially multiple  
>>>>>>> _hours_ of runtime for the defrag).  The performance scaled  
>>>>>>> inversely proportionate to the number of reflinks involved and  
>>>>>>> the total amount of data in the subvolume being defragmented,  
>>>>>>> and was pretty bad even in the case of only a couple of  
>>>>>>> snapshots.
>>>>>>
>>>>>> You cannot ever make the worst program, because an even worse  
>>>>>> program can be made by slowing down the original by a factor of  
>>>>>> 2.
>>>>>> So, you had a badly implemented defrag. At least you got some  
>>>>>> experience. Let's see what went wrong.
>>>>>>
>>>>>>> Ultimately, there are a couple of issues at play here:
>>>>>>>
>>>>>>> * Online defrag has to maintain consistency during operation.   
>>>>>>> The current implementation does this by rewriting the regions  
>>>>>>> being defragmented (which causes them to become a single new  
>>>>>>> extent (most of the time)), which avoids a whole lot of  
>>>>>>> otherwise complicated logic required to make sure things  
>>>>>>> happen correctly, and also means that only the file being  
>>>>>>> operated on is impacted and only the parts being modified need  
>>>>>>> to be protected against concurrent writes.  Properly handling  
>>>>>>> reflinks means that _every_ file that shares some part of an  
>>>>>>> extent with the file being operated on needs to have the  
>>>>>>> reflinked regions locked for the defrag operation, which has a  
>>>>>>> huge impact on performance. Using your example, the update to  
>>>>>>> E55 in both files A and B has to happen as part of the same  
>>>>>>> commit, which can contain no other writes in that region of  
>>>>>>> the file, otherwise you run the risk of losing writes to file  
>>>>>>> B that occur while file A is being defragmented.
>>>>>>
>>>>>> Nah. I think there is a workaround. You can first (atomically)  
>>>>>> update A, then whatever, then you can update B later. I know,  
>>>>>> your yelling "what if E55 gets updated in B". Doesn't matter.  
>>>>>> The defrag continues later by searching for reflink to E55 in  
>>>>>> B. Then it checks the data contained in E55. If the data  
>>>>>> matches the E70, then it can safely update the reflink in B. Or  
>>>>>> the defrag can just verify that neither E55 nor E70 have been  
>>>>>> written to in the meantime. That means they still have the same  
>>>>>> data.
>>>>
>>>>> So, IOW, you don't care if the total space used by the data is  
>>>>> instantaneously larger than what you started with?  That seems  
>>>>> to be at odds with your previous statements, but OK, if we allow  
>>>>> for that then this is indeed a non-issue.
>>>>
>>>> It is normal and common for defrag operation to use some disk  
>>>> space while it is running. I estimate that a reasonable limit  
>>>> would be to use up to 1% of total partition size. So, if a  
>>>> partition size is 100 GB, the defrag can use 1 GB. Lets call this  
>>>> "defrag operation space".
>>>>
>>>> The defrag should, when started, verify that there is "sufficient  
>>>> free space" on the partition. In the case that there is no  
>>>> sufficient free space, the defrag should output the message to  
>>>> the user and abort. The size of "sufficient free space" must be  
>>>> larger than the "defrag operation space". I would estimate that a  
>>>> good limit would be 2% of the partition size. "defrag operation  
>>>> space" is a part of "sufficient free space" while defrag  
>>>> operation is in progress.
>>>>
>>>> If, during defrag operation, sufficient free space drops below  
>>>> 2%, the defrag should output a message and abort. Another  
>>>> possibility is for defrag to pause until the user frees some disk  
>>>> space, but this is not common in other defrag implementations  
>>>> AFAIK.
>>>>
>>>>>>> It's not horrible when it's just a small region in two files,  
>>>>>>> but it becomes a big issue when dealing with lots of files  
>>>>>>> and/or particularly large extents (extents in BTRFS can get  
>>>>>>> into the GB range in terms of size when dealing with really  
>>>>>>> big files).
>>>>>>
>>>>>> You must just split large extents in a smart way. So, in the  
>>>>>> beginning, the defrag can split large extents (2GB) into  
>>>>>> smaller ones (32MB) to facilitate more responsive and easier  
>>>>>> defrag.
>>>>>>
>>>>>> If you have lots of files, update them one-by one. It is  
>>>>>> possible. Or you can update in big batches. Whatever is faster.
>>>>
>>>>> Neither will solve this though.  Large numbers of files are an  
>>>>> issue because the operation is expensive and has to be done on  
>>>>> each file, not because the number of files somehow makes the  
>>>>> operation more espensive. It's O(n) relative to files, not  
>>>>> higher time complexity.
>>>>
>>>> I would say that updating in big batches helps a lot, to the  
>>>> point that it gets almost as fast as defragging any other file  
>>>> system. What defrag needs to do is to write a big bunch of  
>>>> defragged file (data) extents to the disk, and then update the  
>>>> b-trees. What happens is that many of the updates to the b-trees  
>>>> would fall into the same disk sector/extent, so instead of many  
>>>> writes there will be just one write.
>>>>
>>>> Here is the general outline for implementation:
>>>>     - write a big bunch of defragged file extents to disk
>>>>         - a minimal set of updates of the b-trees that cannot be  
>>>> delayed is performed (this is nothing or almost nothing in most  
>>>> circumstances)
>>>>         - put the rest of required updates of b-trees into  
>>>> "pending operations buffer"
>>>>     - analyze the "pending operations buffer", and find out  
>>>> (approximately) the biggest part of it that can be flushed out by  
>>>> doing minimal number of disk writes
>>>>         - flush out that part of "pending operations buffer"
>>>>     - repeat
>>
>>> It helps, but you still can't get around having to recompute the  
>>> new tree state, and that is going to take time proportionate to  
>>> the number of nodes that need to change, which in turn is  
>>> proportionate to the number of files.
>>
>> Yes, but that is just a computation. The defrag performance mostly  
>> depends on minimizing disk I/O operations, not on computations.

> You're assuming the defrag is being done on a system that's  
> otherwise perfectly idle.  In the real world, that rarely, if ever,  
> will be the case,  The system may be doing other things at the same  
> time, and the more computation the defrag operation has to do, the  
> more likely it is to negatively impact those other things.

No, I'm not assuming that the system is perfectly idle. I'm assuming  
that the required computations don't take much CPU time, like it is  
common in a well implemented defrag.

>> In the past many good and fast defrag computation algorithms have  
>> been produced, and I don't see any reason why this project wouldn't  
>> be also able to create such a good algorithm.

> Because it's not just the new extent locations you have to compute,  
> you also need to compute the resultant metadata tree state, and the  
> resultant extent tree state, and after all of that the resultant  
> checksum tree state.  Yeah, figuring out optimal block layouts is  
> solved, but you can't get around the overhead of recomputing the new  
> tree state and all the block checksums for it.
>
> The current defrag has to deal with this too, but it doesn't need to  
> do as much computation because it's not worried about preserving  
> reflinks (and therefore defragmenting a single file won't require  
> updates to any other files).

Yes, the defrag algorithm needs to compute the new tree state.  
However, it shouldn't be slow at all. All operations on b-trees can be  
done in at most N*logN time, which is sufficiently fast. There is no  
operation there that I can think of that takes N*N or N*M time. So, it  
should all take little CPU time. Essentially a non-issue.

The ONLY concern that causes N*M time is the presence of sharing. But,  
even this is unfair, as the computation time will still be N*logN with  
regards to the total number of reflinks. That is still fast, even for  
100 GB metadata with a billion reflinks.

I don't understand why do you think that recomputing the new tree  
state must be slow. Even if there are a 100 new tree states that need  
to be recomputed, there is still no problem. Each metadata update will  
change only a small portion of b-trees, so the complexity and size of  
b-trees should not seriously affect the computation time.

>>>>>> The point is that the defrag can keep a buffer of a "pending  
>>>>>> operations". Pending operations are those that should be  
>>>>>> performed in order to keep the original sharing structure. If  
>>>>>> the defrag gets interrupted, then files in "pending operations"  
>>>>>> will be unshared. But this should really be some important and  
>>>>>> urgent interrupt, as the "pending operations" buffer needs at  
>>>>>> most a second or two to complete its operations.
>>>>
>>>>> Depending on the exact situation, it can take well more than a  
>>>>> few seconds to complete stuff. Especially if there are lots of  
>>>>> reflinks.
>>>>
>>>> Nope. You are quite wrong there.
>>>> In the worst case, the "pending operations buffer" will update  
>>>> (write to disk) all the b-trees. So, the upper limit on time to  
>>>> flush the "pending operations buffer" equals the time to write  
>>>> the entire b-tree structure to the disk (into new extents). I  
>>>> estimate that takes at most a few seconds.
>>
>>> So what you're talking about is journaling the computed state of  
>>> defrag operations.  That shouldn't be too bad (as long as it's  
>>> done in memory instead of on-disk) if you batch the computations  
>>> properly.  I thought you meant having a buffer of what operations  
>>> to do, and then computing them on-the-fly (which would have  
>>> significant overhead)
>>
>> Looks close to what I was thinking. Soon we might be able to  
>> communicate. I'm not sure what you mean by "journaling the computed  
>> state of defrag operations". Maybe it doesn't matter.

> Essentially, doing a write-ahead log of pending operations.   
> Journaling is just the common term for such things when dealing with  
> Linux filesystems because of ext* and XFS.  Based on what you say  
> below, it sounds like we're on the same page here other than the  
> terminology.
>>
>> What happens is that file (extent) data is first written to disk  
>> (defragmented), but b-tree is not immediately updated. It doesn't  
>> have to be. Even if there is a power loss, nothing happens.
>>
>> So, the changes that should be done to the b-trees are put into  
>> pending-operations-buffer. When a lot of file (extent) data is  
>> written to disk, such that defrag-operation-space (1 GB) is close  
>> to being exhausted, the pending-operations-buffer is examined in  
>> order to attempt to free as much of defrag-operation-space as  
>> possible. The simplest algorithm is to flush the entire  
>> pending-operations-buffer at once. This reduces the number of  
>> writes that update the b-trees because many changes to the b-trees  
>> fall into the same or neighbouring disk sectors.
>>
>>>>>>> * Reflinks can reference partial extents.  This means,  
>>>>>>> ultimately, that you may end up having to split extents in odd  
>>>>>>> ways during defrag if you want to preserve reflinks, and might  
>>>>>>> have to split extents _elsewhere_ that are only tangentially  
>>>>>>> related to the region being defragmented. See the example in  
>>>>>>> my previous email for a case like this, maintaining the shared  
>>>>>>> regions as being shared when you defragment either file to a  
>>>>>>> single extent will require splitting extents in the other file  
>>>>>>> (in either case, whichever file you don't defragment to a  
>>>>>>> single extent will end up having 7 extents if you try to force  
>>>>>>> the one that's been defragmented to be the canonical  
>>>>>>> version).  Once you consider that a given extent can have  
>>>>>>> multiple ranges reflinked from multiple other locations, it  
>>>>>>> gets even more complicated.
>>>>>>
>>>>>> I think that this problem can be solved, and that it can be  
>>>>>> solved perfectly (the result is a perfectly-defragmented file).  
>>>>>> But, if it is so hard to do, just skip those problematic  
>>>>>> extents in initial version of defrag.
>>>>>>
>>>>>> Ultimately, in the super-duper defrag, those  
>>>>>> partially-referenced extents should be split up by defrag.
>>>>>>
>>>>>>> * If you choose to just not handle the above point by not  
>>>>>>> letting defrag split extents, you put a hard lower limit on  
>>>>>>> the amount of fragmentation present in a file if you want to  
>>>>>>> preserve reflinks.  IOW, you can't defragment files past a  
>>>>>>> certain point.  If we go this way, neither of the two files in  
>>>>>>> the example from my previous email could be defragmented any  
>>>>>>> further than they already are, because doing so would require  
>>>>>>> splitting extents.
>>>>>>
>>>>>> Oh, you're reading my thoughts. That's good.
>>>>>>
>>>>>> Initial implementation of defrag might be not-so-perfect. It  
>>>>>> would still be better than the current defrag.
>>>>>>
>>>>>> This is not a one-way street. Handling of partially-used  
>>>>>> extents can be improved in later versions.
>>>>>>
>>>>>>> * Determining all the reflinks to a given region of a given  
>>>>>>> extent is not a cheap operation, and the information may  
>>>>>>> immediately be stale (because an operation right after you  
>>>>>>> fetch the info might change things).  We could work around  
>>>>>>> this by locking the extent somehow, but doing so would be  
>>>>>>> expensive because you would have to hold the lock for the  
>>>>>>> entire defrag operation.
>>>>>>
>>>>>> No. DO NOT LOCK TO RETRIEVE REFLINKS.
>>>>>>
>>>>>> Instead, you have to create a hook in every function that  
>>>>>> updates the reflink structure or extents (for exaple,  
>>>>>> write-to-file operation). So, when a reflink gets changed, the  
>>>>>> defrag is immediately notified about this. That way the defrag  
>>>>>> can keep its data about reflinks in-sync with the filesystem.
>>>>
>>>>> This doesn't get around the fact that it's still an expensive  
>>>>> operation to enumerate all the reflinks for a given region of a  
>>>>> file or extent.
>>>>
>>>> No, you are wrong.
>>>>
>>>> In order to enumerate all the reflinks in a region, the defrag  
>>>> needs to have another array, which is also kept in memory and in  
>>>> sync with the filesystem. It is the easiest to divide the disk  
>>>> into regions of equal size, where each region is a few MB large.  
>>>> Lets call this array "regions-to-extents" array. This array  
>>>> doesn't need to be associative, it is a plain array.
>>>> This in-memory array links regions of disk to extents that are in  
>>>> the region. The array in initialized when defrag starts.
>>>>
>>>> This array makes the operation of finding all extents of a region  
>>>> extremely fast.
>>> That has two issues:
>>>
>>> * That's going to be a _lot_ of memory.  You still need to be able  
>>> to defragment big (dozens plus TB) arrays without needing multiple  
>>> GB of RAM just for the defrag operation, otherwise it's not  
>>> realistically useful (remember, it was big arrays that had issues  
>>> with the old reflink-aware defrag too).
>>
>> Ok, but let's get some calculations there. If regions are 4 MB in  
>> size, the region-extents array for an 8 TB partition would have 2  
>> million entries. If entries average 64 bytes, that would be:
>>
>>  - a total of 128 MB memory for an 8 TB partition.
>>
>> Of course, I'm guessing a lot of numbers there, but it should be doable.

> Even if we assume such an optimistic estimation as you provide (I  
> suspect it will require more than 64 bytes per-entry), that's a lot  
> of RAM when you look at what it's potentially displacing.  That's  
> enough RAM for receive and transmit buffers for a few hundred  
> thousand network connections, or for caching multiple hundreds of  
> thousands of dentries, or a few hundred thousand inodes.  Hell,  
> that's enough RAM to run all the standard network services for a  
> small network (DHCP, DNS, NTP, TFTP, mDNS relay, UPnP/NAT-PMP, SNMP,  
> IGMP proxy, VPN of your choice) at least twice over.

That depends on the average size of an extent. If the average size of  
an extent is around 4 MB, than my numbers should be good. Do you have  
any data which would suggest that my estimate is wrong? What's the  
average size of an extent on your filesystems (used space divided by  
number of extents)?
This "regions-to-extents" array can be further optimized if necessary.

You are not thinking correctly there (misplaced priorities). If the  
system needs to be defragmented, that's the priority. You can't do  
comparisons like that, that's unfair debating.

The defrag that I'm proposing should be able to run within common  
memory limits of today's computer systems. So, it will likely take  
somewhat less than 700 MB of RAM in most common situations, including  
the small servers. They all have 700 MB RAM.

700 MB is a lot for a defrag, but there is no way around it. Btrfs is  
simply a filesystem with such complexity that a good defrag requires a  
lot of RAM to operate.

If, for some reason, you would like to cover a use-case with  
constrained RAM conditions, then that is an entirely different concern  
for a different project. You can't make a project like this to cover  
ALL the possible circumstances. Some cases have to be left out. Here  
we are talking about a defrag that is usable in a general and common  
set of circumstances.

Please, don't drop special circumstances argument on me. That's not fair.

>>> * You still have to populate the array in the first place.  A sane  
>>> implementation wouldn't be keeping it in memory even when defrag  
>>> is not running (no way is anybody going to tolerate even dozens of  
>>> MB of memory overhead for this), so you're not going to get around  
>>> the need to enumerate all the reflinks for a file at least once  
>>> (during startup, or when starting to process that file), so you're  
>>> just moving the overhead around instead of eliminating it.
>>
>> Yes, when the defrag starts, the entire b-tree structure is  
>> examined in order for region-extents array and extents-backref  
>> associative array to be populated.

> So your startup is going to take forever on any reasonably large  
> volume.  This isn't eliminating the overhead, it's just moving it  
> all to one place.  That might make it a bit more efficient than it  
> would be interspersed throughout the operation, but only because it  
> is reading all the relevant data at once.

No, the startup will not take forever.

The startup needs exactly 1 (one) pass through the entire metadata. It  
needs this to find all the backlinks and to populate the  
"regios-extents" array.  The time to do 1 pass through metadata  
depends on the metadata size on disk, as entire metadata has to be  
read out (one piece at a time, you won't keep it all in RAM). In most  
cases, the time-to read the metadata will be less than 1 minute, on an  
SSD less than 20 seconds.

There is no way around it: to defrag, you eventually need to read all  
the b-trees, so nothing is lost there.

All computations in this defrag are simple. Finding all refliks in  
metadata is simple. It is a single pass metadata read-out.

>> Of course, those two arrays exist only during defrag operation.  
>> When defrag completes, those arrays are deallocated.
>>
>>>>> It also allows a very real possibility of a user functionally  
>>>>> delaying the defrag operation indefinitely (by triggering a  
>>>>> continuous stream of operations that would cause reflink changes  
>>>>> for a file being operated on by defrag) if not implemented very  
>>>>> carefully.
>>>>
>>>> Yes, if a user does something like that, the defrag can be paused  
>>>> or even aborted. That is normal.
>>> Not really.  Most defrag implementations either avoid files that  
>>> could reasonably be written to, or freeze writes to the file  
>>> they're operating on, or in some other way just sidestep the issue  
>>> without delaying the defragmentation process.
>>>>
>>>> There are many ways around this problem, but it really doesn't  
>>>> matter, those are just details. The initial version of defrag can  
>>>> just abort. The more mature versions of defrag can have a better  
>>>> handling of this problem.
>>
>>> Details like this are the deciding factor for whether something is  
>>> sanely usable in certain use cases, as you have yourself found out  
>>> (for a lot of users, the fact that defrag can unshare extents is  
>>> 'just a detail' that's not worth worrying about).
>>
>> I wouldn't agree there.
>>
>> Not every issue is equal. Some issues are more important, some are  
>> trivial, some are tolerable etc...
>>
>> The defrag is usually allowed to abort. It can easily be restarted  
>> later. Workaround: You can make a defrag-supervisor program, which  
>> starts a defrag, and if defrag aborts then it is restarted after  
>> some (configurable) amount of time.

> The fact that the defrag can be functionally deferred indefinitely  
> by a user means that a user can, with a bit of effort, force  
> degraded performance for everyone using the system.  Aborting the  
> defrag doesn't solve that, and it's a significant issue for anybody  
> doing shared hosting.

This is a quality-of-implementation issue. Not worthy of consideration  
at this time. It can be solved.

You can go and pick this kind of stuff all the time, with any system.  
I mean, because of the FACT that we have never proven that all  
security holes are eliminated, the computers shouldn't be powered on  
at all. Therefore, all computers should be shut down immediately and  
then there is absolutely no need to continue working on the btrfs. It  
is also impossible to produce the btrfs defrag, because all computers  
have to be shut down immediately.

Can we have a bit more fair discussion? Please?

>>
>> On the other hand, unsharing is not easy to get undone.
> But, again, it this just doesn't matter for some people.
>>
>> So, those issues are not equals.