Re: XFS reflink vs ThinLVM

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

Il 15-01-2020 17:39 Darrick J. Wong ha scritto:
> extszinherit > 0 disables delayed allocation, which means that (in your
> case above) if you wrote 1G to a file (using the pagecache) you'd get
> 8192x 128K calls to the allocator instead of making a single 1G
> allocation during writeback.

Thanks for the valuable information, I did not know that specific 
interaction between extsize and delalloc.

> If you have a lot of memory (or a high vmm
> dirty ratio) then you want delalloc over extsize.  Most of the time you
> want delalloc, frankly.

Let me briefly describe the expected workload: thinly provisioned 
virtual image storage. The problem with "plain" sparse file (ie: without 
extsize hint) is that, after some time, the underlying vdisk file will 
be very fragmented: consecutive physical blocks will be assigned to very 
different logical blocks, leading to sub-par performance when reading 
back the whole file (eg: for backup purpose).

I can easily simulate a worst-case scenario with fio, issuing random 
write to a pre-created sparse file. While the random writes complete 
very fast (because they are more-or-less sequentially written inside the 
sparse file), reading back that file will have very low performance: 10 
MB/s vs 600+ MB/s for a preallocated file.

Using a 128k extsize brings sequential read to ~100 MB/s (which is 
reasonable on that old hardware), and a 16M extsize is in the range of 
500+ MB/s.

Given that use case, do you suggest sticking with delalloc or setting an 
appropriate extsize?

Thanks.

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Danti Gionatan
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