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

[webmaster@zedlx.com]

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.

> 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.

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.

> * 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.

Also note, this defrag should run as a part of the kernel, not in  
userspace. Defrag-from-userspace is a nightmare. Defrag has to  
serialize its operations properly, and it must have knowledge of all  
other operations in progress. So, it can only operate efficiently as  
part of the kernel.