Re: XFS reflink vs ThinLVM

[Gionatan Danti <g.danti@assyoma.it>]

On 13/01/20 13:21, Gionatan Danti wrote:
> On 13/01/20 12:43, Carlos Maiolino wrote:
>> I should have mentioned it, my apologies.
>>
>> 'extsize' argument for mkfs.xfs will set the size of the blocks in the RT
>> section.
>>
>> Although, the 'extsize' command in xfs_io, will set the extent size 
>> hints on any
>> file of any xfs filesystem (or filesystem supporting FS_IOC_FSSETXATTR).
>>
>> Notice you can use xfs_io extsize to set the extent size hint to a 
>> directory,
>> and all files under the directory will inherit the same extent hint.
> 
> My bad, I forgot about xfs_io.
> Thanks for the detailed explanation.

Well, I did some test with a reflinked file and I must say I am 
impressed on how well XFS handles small rewrites (for example 4K).

 From my understanding, by mapping at 4K granularity but allocating at 
128K, it avoid most read/write amplification *and* keep low 
fragmentation. After "speculative_cow_prealloc_lifetime" it reclaim the 
allocated but unused space, bringing back any available free space to 
the filesystem. Is this understanding correct?

I have a question: how can I see the allocated-but-unused cow extents? 
For example, giving the following files:

[root@neutron xfs]# stat test.img copy.img
   File: test.img
   Size: 1073741824      Blocks: 2097400    IO Block: 4096   regular file
Device: 810h/2064d      Inode: 131         Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Context: unconfined_u:object_r:unlabeled_t:s0
Access: 2020-01-13 15:40:50.280711297 +0100
Modify: 2020-01-13 16:21:55.564726283 +0100
Change: 2020-01-13 16:21:55.564726283 +0100
  Birth: -

   File: copy.img
   Size: 1073741824      Blocks: 2097152    IO Block: 4096   regular file
Device: 810h/2064d      Inode: 132         Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Context: unconfined_u:object_r:unlabeled_t:s0
Access: 2020-01-13 15:40:50.280711297 +0100
Modify: 2020-01-13 15:40:57.828552412 +0100
Change: 2020-01-13 15:41:48.190492279 +0100
  Birth: -

I can clearly see that test.img has an additional 124K allocated after a 
4K rewrite. This matches my expectation: a 4K rewrite really allocates a 
128K blocks, leading to 124K of temporarily "wasted" space.

But both "filefrag -v" and "xfs_bmap -vep" show only the used space as 
seen by an userspace application (ie: 262144 blocks of 4096 bytes = 
1073741824 bytes).

How can I check the total allocated space as reported by stat?
Thanks.

-- 
Danti Gionatan
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Assyoma S.r.l. - www.assyoma.it
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