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

Austin S. Hemmelgarn
2019-09-12T19:54:40Z

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.
>
>>> 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)
>
>>>> * 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).
* 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.
>
>> 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).