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

[Alberto Garcia <berto@igalia.com>]

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

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.

Berto