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

General Zed
2019-09-13T09:25:29Z

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.