Re: Is there any way to determine fragmentation for compressed btrfs volumes?

[Clemens Eisserer <linuxhippy@gmail.com>]

Hi again,

So it seems performance issues caused by FS degredation are in the
current development stage of btrfs not getting a lot of attention.
Hopefully production use on facebook servers will expose one or
another issue and with the developers being employed there chances are
good btrfs will handle workload like mine more gracefully with these
issues fixed.

Thanks & best regards, Clemens

2014-04-08 21:41 GMT+02:00 Clemens Eisserer <linuxhippy@gmail.com>:
> Hi Ducan,
>
>> You mention trying scrub and defragging the entire volume, but you don't
>> mention balance.  Balance by default rewrites all chunks (tho you can add
>> filters to rewrite only say data chunks, not metadata, if you like), so
>> that's what I'd say to try, as it should defrag in the process.
>>
>> Tho we've seen a few reports from people saying a full balance actually
>> made for instance boot times longer instead of shorter, too.  I haven't
>> seen and don't have an explanation for that. <shrug>
>
> Ah sorry, I confused scrub with balance.
> I didn't do a scrub but actually I tried to balance the device as I
> was told basically all data has to go through the allocator again
> which most likely will improve on-disk layout. Unfourtunatly in my
> case it made the situation a lot worse, when running this Linux system
> in virtualbox (where disk access has a lot more overhead) it is now
> even with the SSD super slow.
>
>
>> Meanwhile, have you tried a trim/discard (fstrim command), and/or do you
>> run with the discard (or is it trim) mount option?
> I have tried with discard on and off - however this isn't a SSD issue.
>
> It is slow even for read-only workload, and windows on the same SSD
> works as expected (with CrystalDiskMark showing the dirve meets the
> specs (~350mb/s seq. write, 500b/s sequential read, good 4k random
> values).
> So I doubt this is an SSD issue.
>
>
>> One more thing to consider.  The btrfs of a year and a half ago was a
>> rather less mature btrfs than we have today.  I recently booted to backup
>> here, and did a brand new mkfs.btrfs on my working filesystems to take
>> advantage of several of the newer features
>
> I had hopes I could avoid that.
> Having a FS in such a degraded state with the only option to re-create
> it isn't that compelling ;)
>
> Thanks & regards, Clemens