mkfs.xfs: cannot (re)set log sunit=0
Hello,
I intend to set up a XFS filesystem on a RAID0 (Linux md) with 512k Chunk Size. Since I cannot align the log section sunit to 512k, and since the filesystem will have not much write activity anyways (in fact, it'll be mounted read-only most of the time), I tried not to align the log section sunit at all via -l sunit=0, but this does not appear to work:
# truncate -s 1G /tmp/foo{0,1}
# losetup /dev/loop0 /tmp/foo0; losetup /dev/loop1 /tmp/foo1
# mdadm -C -l raid0 -n 2 /dev/md9 /dev/loop[01]
# mkfs.xfs -l sunit=0 /dev/md9
log stripe unit (524288 bytes) is too large (maximum is 256KiB)
log stripe unit adjusted to 32KiB
meta-data=/dev/md9 isize=256 agcount=8, agsize=65408 blks
= sectsz=512 attr=2
data = bsize=4096 blocks=523264, imaxpct=25
= sunit=128 swidth=256 blks
naming =version 2 bsize=4096 ascii-ci=0
log =internal log bsize=4096 blocks=2560, version=2
= sectsz=512 sunit=8 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0Is this a bug or a feature?
The minimum I was able to limit the log sunit to was -l sunit=8, i.e. 1 block. Is there a difference between sunit=0 and sunit=1 in practice or are single log entries aligned to blocks anyways?
Btw...
Since the filesystem is mostly read-only, I was also thinking about reducing the size of the log section. The planned filesystem will be 5.5T or bigger, which results in 2G log section per default. I thought about limiting it to 128M or 64M. There will be no concurrent write activity on the fs. Together with the zero-alignment, what do you think about that?
PS: I'm not on this list, please CC: me in replies. My Mail-Followup-To:
header should take care of that.
Thanks for your help & regards
Mario
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Quoted 8 lines · Mario 'BitKoenig' Holbe
Hello,
I intend to set up a XFS filesystem on a RAID0 (Linux md) with 512k Chunk Size. Since I cannot align the log section sunit to 512k, and since the filesystem will have not much write activity anyways (in fact, it'll be mounted read-only most of the time), I tried not to align the log section sunit at all via -l sunit=0, but this does not appear to work:
Of course - when you specific a value yourself, it has to be a valid value and for version 2 logs that minimum value for th lsunit is one log block. Given that:
....
Quoted 1 line
log =internal log bsize=4096 blocks=2560, version=2
....
^^^^^^^^^^
Your log block size is 4k, then:
Quoted 4 lines
Is this a bug or a feature?
The minimum I was able to limit the log sunit to was -l sunit=8, i.e. 1 block.
This is the correct minimum you are allowed to specify.
Quoted 2 lines
Is there a difference between sunit=0 and sunit=1 in practice or are single log entries aligned to blocks anyways?
sunit=0 is an invalid configuration for a version 2 log. Filesystems without a data sunit default to a lsunit = 1 log block, so in practice version 2 logs are always aligned/padded in some way.
Quoted 7 lines
Btw...
Since the filesystem is mostly read-only, I was also thinking about reducing the size of the log section. The planned filesystem will be 5.5T or bigger, which results in 2G log section per default. I thought about limiting it to 128M or 64M. There will be no concurrent write activity on the fs. Together with the zero-alignment, what do you think about that?
That'll be fine - the maximum log size until a couple of years ago was 128MB, and I still do most of my benchmarking with a log that size....
Cheers,
Dave.
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