Re: [PATCH 0/1] qcow2: Skip copy-on-write when allocating a zero cluster

[Brian Foster <bfoster@redhat.com>]

On Tue, Aug 25, 2020 at 07:18:19PM +0200, Alberto Garcia wrote:
> On Tue 25 Aug 2020 06:54:15 PM CEST, Brian Foster wrote:
> > If I compare this 5m fio test between XFS and ext4 on a couple of my
> > systems (with either no prealloc or full file prealloc), I end up seeing
> > ext4 run slightly faster on my vm and XFS slightly faster on bare metal.
> > Either way, I don't see that huge disparity where ext4 is 5-6 times
> > faster than XFS. Can you describe the test, filesystem and storage in
> > detail where you observe such a discrepancy?
> 
> Here's the test:
> 
> fio --filename=/path/to/file.raw --direct=1 --randrepeat=1 \
>     --eta=always --ioengine=libaio --iodepth=32 --numjobs=1 \
>     --name=test --size=25G --io_limit=25G --ramp_time=0 \
>     --rw=randwrite --bs=4k --runtime=300 --time_based=1
> 

My fio fallocates the entire file by default with this command. Is that
the intent of this particular test? I added --fallocate=none to my test
runs to incorporate the allocation cost in the I/Os.

> The size of the XFS filesystem is 126 GB and it's almost empty, here's
> the xfs_info output:
> 
> meta-data=/dev/vg/test           isize=512    agcount=4, agsize=8248576
> blks
>          =                       sectsz=512   attr=2, projid32bit=1
>          =                       crc=1        finobt=1, sparse=1,
>          rmapbt=0
>          =                       reflink=0
> data     =                       bsize=4096   blocks=32994304, imaxpct=25
>          =                       sunit=0      swidth=0 blks
> naming   =version 2              bsize=4096   ascii-ci=0, ftype=1
> log      =internal log           bsize=4096   blocks=16110, version=2
>          =                       sectsz=512   sunit=0 blks, lazy-count=1
> realtime =none                   extsz=4096   blocks=0, rtextents=0
> 
> The size of the ext4 filesystem is 99GB, of which 49GB are free (that
> is, without the file used in this test). The filesystem uses 4KB
> blocks, a 128M journal and these features:
> 
> Filesystem revision #:    1 (dynamic)
> Filesystem features:      has_journal ext_attr resize_inode dir_index
>                           filetype needs_recovery extent flex_bg
>                           sparse_super large_file huge_file uninit_bg
>                           dir_nlink extra_isize
> Filesystem flags:         signed_directory_hash
> Default mount options:    user_xattr acl
> 
> In both cases I'm using LVM on top of LUKS and the hard drive is a
> Samsung SSD 850 PRO 1TB.
> 
> The Linux version is 4.19.132-1 from Debian.
> 

Thanks. I don't have LUKS in the mix on my box, but I was running on a
more recent kernel (Fedora 5.7.15-100). I threw v4.19 on the box and saw
a bit more of a delta between XFS (~14k iops) and ext4 (~24k). The same
test shows ~17k iops for XFS and ~19k iops for ext4 on v5.7. If I
increase the size of the LVM volume from 126G to >1TB, ext4 runs at
roughly the same rate and XFS closes the gap to around ~19k iops as
well. I'm not sure what might have changed since v4.19, but care to see
if this is still an issue on a more recent kernel?

Brian

> Berto
>