On 11/26/13, 8:47 PM, Dave Chinner wrote:
> On Tue, Nov 26, 2013 at 06:31:19PM -0800, Phil White wrote:
>> I was making an image for a VM using everyone's favorite fs with a line
>> that looked something like this:
>> dd if=/dev/zero of=~/image bs=1024 count=1048576 && ./mkfs/mkfs.xfs && mount
>> -o loop ~/image /mnt/loop
>> mkfs.xfs gave me this output:
>> meta-data=/root/image isize=256 agcount=4, agsize=65536 blks
>> = sectsz=512 attr=2, projid32bit=0
>> data = bsize=4096 blocks=262144, imaxpct=25
>> = sunit=0 swidth=0 blks
>> naming =version 2 bsize=4096 ascii-ci=0
>> log =internal log bsize=4096 blocks=2560, version=2
>> = sectsz=512 sunit=0 blks, lazy-count=1
>> realtime =none extsz=4096 blocks=0, rtextents=0
>> existing superblock read failed: Invalid argument
>> mkfs.xfs: pwrite64 failed: Invalid argument
>> mkfs.xfs: read failed: Invalid argument
>> While it occurred to me that the problem might just be line 806 of some files
>> in xfsprogs, I threw it under a debugger and took a closer look. The file
>> descriptor value in xi->dfd pointed at ~/image. errno was set to 22. I
>> thought that might indicate a problem with lseek(), so I rewrote the
>> and pread() as lseek()s and read()/write()
>> As you may have guessed, this did me no good at all.
>> It's trying to read/write 512 bytes at the beginning of the file which seems
>> reasonably innocuous. I double checked the man page which says that under
>> 2.6, O_DIRECT writes can be aligned to 512 bytes without a problem.
> That doesn't mean it is correct, because the man page also says:
> " In Linux alignment restrictions vary by filesystem and kernel
> version and might be absent entirely."
> So, I bet that your underlying filesystem (i.e. the host filesystem)
> has a sector size of 4k, and that's why direct Io on 512 byte
> alignment is failing. In that case, run "mkfs.xfs -s size=4k ..."
> and mkfs should just work fine...
Sadly, no. Or at least, probably not.
where libxfs_device_alignment() does:
and through twisty paths through platform_findsizes(), max_block_alignment is
BBSIZE, or 512.
IOWS: xfsprogs is a braindead package that doesn't know how to properly handle
non-512-aligned DIO. ;) </snark>
We should fix xfsprogs, but I'm also looking making 4k "sector" xfs able to do
512 DIOs as long as the device under it can do 512 DIOS.
We got into the 4k DIO alignment because mkfs.xfs saw a 512 logical / 4k
physical drive, and chose 4k for its own internal sector size, which is great
in terms of doing metadata IO efficiently, but not so great in terms of
rejecting 512 byte IOs that the drive could otherwise do.