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

General Zed
2019-09-13T17:02:43Z

Quoting General Zed <general-zed@zedlx.com>:

> Quoting General Zed <general-zed@zedlx.com>:
>
>> Quoting Zygo Blaxell <ce3g8jdj@umail.furryterror.org>:
>>
>>> On Thu, Sep 12, 2019 at 08:26:04PM -0400, General Zed wrote:
>>>>
>>>> Quoting Zygo Blaxell <ce3g8jdj@umail.furryterror.org>:
>>>>
>>>>> On Thu, Sep 12, 2019 at 06:57:26PM -0400, General Zed wrote:
>>>>>>
>>>>>> At worst, it just has to completely write-out "all metadata",
>>>>>> all the way up
>>>>>> to the super. It needs to be done just once, because what's the point of
>>>>>> writing it 10 times over? Then, the super is updated as the
>>>>>> final commit.
>>>>>
>>>>> This is kind of a silly discussion. The biggest extent possible on
>>>>> btrfs is 128MB, and the incremental gains of forcing 128MB extents to
>>>>> be consecutive are negligible. If you're defragging a 10GB file, you're
>>>>> just going to end up doing 80 separate defrag operations.
>>>>
>>>> Ok, then the max extent is 128 MB, that's fine. Someone here
>>>> previously said
>>>> that it is 2 GB, so he has disinformed me (in order to further his false
>>>> argument).
>>>
>>> If the 128MB limit is removed, you then hit the block group size limit,
>>> which is some number of GB from 1 to 10 depending on number of disks
>>> available and raid profile selection (the striping raid profiles cap
>>> block group sizes at 10 disks, and single/raid1 profiles always use 1GB
>>> block groups regardless of disk count). So 2GB is _also_ a valid extent
>>> size limit, just not the first limit that is relevant for defrag.
>>>
>>> A lot of people get confused by 'filefrag -v' output, which coalesces
>>> physically adjacent but distinct extents. So if you use that tool,
>>> it can _seem_ like there is a 2.5GB extent in a file, but it is really
>>> 20 distinct 128MB extents that start and end at adjacent addresses.
>>> You can see the true structure in 'btrfs ins dump-tree' output.
>>>
>>> That also brings up another reason why 10GB defrags are absurd on btrfs:
>>> extent addresses are virtual. There's no guarantee that a pair of extents
>>> that meet at a block group boundary are physically adjacent, and after
>>> operations like RAID array reorganization or free space defragmentation,
>>> they are typically quite far apart physically.
>>>
>>>> I didn't ever said that I would force extents larger than 128 MB.
>>>>
>>>> If you are defragging a 10 GB file, you'll likely have to do it
>>>> in 10 steps,
>>>> because the defrag is usually allowed to only use a limited amount of disk
>>>> space while in operation. That has nothing to do with the extent size.
>>>
>>> Defrag is literally manipulating the extent size. Fragments and extents
>>> are the same thing in btrfs.
>>>
>>> Currently a 10GB defragment will work in 80 steps, but doesn't necessarily
>>> commit metadata updates after each step, so more than 128MB of temporary
>>> space may be used (especially if your disks are fast and empty,
>>> and you start just after the end of the previous commit interval).
>>> There are some opportunities to coalsce metadata updates, occupying up
>>> to a (arbitrary) limit of 512MB of RAM (or when memory pressure forces
>>> a flush, whichever comes first), but exploiting those opportunities
>>> requires more space for uncommitted data.
>>>
>>> If the filesystem starts to get low on space during a defrag, it can
>>> inject commits to force metadata updates to happen more often, which
>>> reduces the amount of temporary space needed (we can't delete the original
>>> fragmented extents until their replacement extent is committed); however,
>>> if the filesystem is so low on space that you're worried about running
>>> out during a defrag, then you probably don't have big enough contiguous
>>> free areas to relocate data into anyway, i.e. the defrag is just going to
>>> push data from one fragmented location to a different fragmented location,
>>> or bail out with "sorry, can't defrag that."
>>
>> Nope.
>>
>> Each defrag "cycle" consists of two parts:
>> 1) move-out part
>> 2) move-in part
>>
>> The move-out part select one contiguous area of the disk. Almost
>> any area will do, but some smart choices are better. It then
>> moves-out all data from that contiguous area into whatever holes
>> there are left empty on the disk. The biggest problem is actually
>> updating the metadata, since the updates are not localized.
>> Anyway, this part can even be skipped.
>>
>> The move-in part now populates the completely free contiguous area
>> with defragmented data.
>>
>> In the case that the move-out part needs to be skipped because the
>> defrag estimates that the update to metatada will be too big (like
>> in the pathological case of a disk with 156 GB of metadata), it can
>> sucessfully defrag by performing only the move-in part. In that
>> case, the move-in area is not free of data and "defragmented" data
>> won't be fully defragmented. Also, there should be at least 20%
>> free disk space in this case in order to avoid defrag turning
>> pathological.
>>
>> But, these are all some pathological cases. They should be
>> considered in some other discussion.
>
> I know how to do this pathological case. Figured it out!
>
> Yeah, always ask General Zed, he knows the best!!!
>
> The move-in phase is not a problem, because this phase generally
> affects a low number of files.
>
> So, let's consider the move-out phase. The main concern here is that
> the move-out area may contain so many different files and fragments
> that the move-out forces a practically undoable metadata update.
>
> So, the way to do it is to select files for move-out, one by one (or
> even more granular, by fragments of files), while keeping track of
> the size of the necessary metadata update. When the metadata update
> exceeds a certain amount (let's say 128 MB, an amount that can
> easily fit into RAM), the move-out is performed with only currently
> selected files (file fragments). (The move-out often doesn't affect
> a whole file since only a part of each file lies within the move-out
> area).
>
> Now the defrag has to decide: whether to continue with another round
> of the move-out to get a cleaner move-in area (by repeating the same
> procedure above), or should it continue with a move-in into a
> partialy dirty area. I can't tell you what's better right now, as
> this can be determined only by experiments.
>
> Lastly, the move-in phase is performed (can be done whether the
> move-in area is dirty or completely clean). Again, the same trick
> can be used: files can be selected one by one until the calculated
> metadata update exceeds 128 MB. However, it is more likely that the
> size of move-in area will be exhausted before this happens.
>
> This algorithm will work even if you have only 3% free disk space left.
>
> This algorithm will also work if you have metadata of huge size, but
> in that case it is better to have much more free disk space (20%) to
> avoid significantly slowing down the defrag operation.

I have just thought out an even better algorithm than this which gets
to fully-defragged state faster, in a smaller number of disk writes.
But I won't write it down unless someone says thanks for your effort
so far, General Zed, and can you please tell us about your great new
defrag algorithm for low free-space conditions.