On Wed, Nov 08, 2017 at 01:21:18PM -0800, L A Walsh wrote:
> Dave Chinner wrote:
> >Are you still getting all worked up about how metadata CRCs and
> >the v5 on-disk format is going to make the sky fall, Linda? It's
> >time to give in and come join us on the dark side...
> ---
> I don't believe I've heard that the sky would fall. I only had
> 2 issues -- 1 that metadata that I that I didn't care about or that I
> wanted to change would be crc'd and prevent changing meta data I wanted
> to change or would flag errors in meta data I didn't care about
> (file last access time being a nanosecond or a day off due to bit rot
> and crc flagging it as an error.
>
> Maybe you might remember, I first ran into this when, as part of
> my mkfs procedure, I assigned my own value to my disk's UUID, and at the
> time, the crc-feature claimed the disk had a fault in it.
Yes, but changing the UUID was documented as "not currently
supported" on v5 filesystems *when it was originally released*.
IOWs, it was documented as "will be supported in future", but it
wasn't a critical feature for the initial release of CRC enabled
filesystems.
If someone manually changed the UUID (which was the only way to do
it because the xfs_db commands would refuse to do it) then *it broke
the filesystem* and so it was correct behaviour to report
corruption.
Changing the UUID on v5 filesystems is now implemented and
supported:
$ sudo mkfs.xfs -f /dev/pmem0
Default configuration sourced from package build definitions
meta-data=/dev/pmem0 isize=512 agcount=4, agsize=524288 blks
= sectsz=4096 attr=2, projid32bit=1
= crc=1 finobt=1, sparse=0, rmapbt=0, reflink=0
data = bsize=4096 blocks=2097152, imaxpct=25, thinblocks=0
= sunit=0 swidth=0 blks
naming =version 2 bsize=4096 ascii-ci=0 ftype=1
log =internal log bsize=4096 blocks=2560, version=2
= sectsz=4096 sunit=1 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0
$
$ sudo blkid -c /dev/null /dev/pmem0
/dev/pmem0: UUID="7073fe11-4b44-4160-a8a0-dec492f61a14" TYPE="xfs"
$
$ sudo xfs_admin -U generate /dev/pmem0
Clearing log and setting UUID
writing all SBs
new UUID = c3a4f999-b76a-4597-bb62-df11c5e3fc04
$
$ sudo blkid -c /dev/null /dev/pmem0
/dev/pmem0: UUID="c3a4f999-b76a-4597-bb62-df11c5e3fc04" TYPE="xfs"
$
IOWs, this problem is ancient history. Move on, nothing to see here.
> My second issue was it being tied to the finobt feature in a way that
> precluded benchmarking changes on our own filesystems and workload.
[....]
> I would expect that, especially since finobt would benefit more mature
> file systems more than newer ones. While on newer file systems, finobt+crc
> comes out to about the same performance.
>
> My issue was the inability to bench or use them separately.
<sigh>
Not an XFS problem:
$ mkfs.xfs -f -m finobt=0 /dev/pmem0
....
= crc=1 finobt=0, sparse=0, rmapbt=0, reflink=0
.....
Yup, crc's enabled, finobt is not. As documented in the mkfs.xfs man
page.
IOWs, We can directly measure the impact of the finobt on
workloads/benchamrks. And if we want to compare the impact of CRCs,
then 'mkfs.xfs -f -isize=512, -m crc=0 <dev>' will be directly
comparable to the above non-finobt filesystem. THis is how we
benchmarked the changes in the first place....
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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