On Wed, Apr 03, 2019 at 02:19:11PM +0100, Luis Henriques wrote:
> Nikolay Borisov <nborisov@xxxxxxxx> writes:
> > On 3.04.19 г. 12:45 ч., Luis Henriques wrote:
> >> Dave Chinner <david@xxxxxxxxxxxxx> writes:
> >>> Makes no sense to me. xfs_io does a write() loop internally with
> >>> this pwrite command of 4kB writes - the default buffer size. If you
> >>> want xfs_io to loop doing 1MB sized pwrite() calls, then all you
> >>> need is this:
> >>>
> >>> $XFS_IO_PROG -f -c "pwrite -w -B 1m 0 ${size}m" $file | _filter_xfs_io
> >>>
> >>
> >> Thank you for your review, Dave. I'll make sure the next revision of
> >> these tests will include all your comments implemented... except for
> >> this one.
> >>
> >> The reason I'm using a loop for writing a file is due to the nature of
> >> the (very!) loose definition of quotas in CephFS. Basically, clients
> >> will likely write some amount of data over the configured limit because
> >> the servers they are communicating with to write the data (the OSDs)
> >> have no idea about the concept of quotas (or files even); the filesystem
> >> view in the cluster is managed at a different level, with the help of
> >> the MDS and the client itself.
> >>
> >> So, the loop in this function is simply to allow the metadata associated
> >> with the file to be updated while we're writing the file. If I use a
> >
> > But the metadata will be modified while writing the file even with a
> > single invocation of xfs_io.
>
> No, that's not true. It would be too expensive to keep the metadata
> server updated while writing to a file. So, making sure there's
> actually an open/close to the file (plus the fsync in pwrite) helps
> making sure the metadata is flushed into the MDS.
/me sighs.
So you want:
loop until ${size}MB written:
write 1MB
fsync
-> flush data to server
-> flush metadata to server
i.e. this one liner:
xfs_io -f -c "pwrite -D -B 1m 0 ${size}m" /path/to/file
Fundamentally, if you find yourself writing a loop around xfs_io to
break up a sequential IO stream into individual chunks, then you are
most likely doing something xfs_io can already do. And if xfs_io
cannot do it, then the right thing to do is to modify xfs_io to be
able to do it and then use xfs_io....
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
