Hi Joseph,
On 26/01/16 09:07, Joseph Fernandes wrote:
Answer inline:
----- Original Message -----
From: "Xavier Hernandez" <xhernandez@xxxxxxxxxx>
To: "Pranith Kumar Karampuri" <pkarampu@xxxxxxxxxx>, "Gluster Devel" <gluster-devel@xxxxxxxxxxx>
Sent: Tuesday, January 26, 2016 1:21:37 PM
Subject: Re: distributed files/directories and [cm]time updates
Hi Pranith,
On 26/01/16 03:47, Pranith Kumar Karampuri wrote:
hi,
Traditionally gluster has been using ctime/mtime of the
files/dirs on the bricks as stat output. Problem we are seeing with this
approach is that, software which depends on it gets confused when there
are differences in these times. Tar especially gives "file changed as we
read it" whenever it detects ctime differences when stat is served from
different bricks. The way we have been trying to solve it is to serve
the stat structures from same brick in afr, max-time in dht. But it
doesn't avoid the problem completely. Because there is no way to change
ctime at the moment(lutimes() only allows mtime, atime), there is little
we can do to make sure ctimes match after self-heals/xattr
updates/rebalance. I am wondering if anyone of you solved these problems
before, if yes how did you go about doing it? It seems like applications
which depend on this for backups get confused the same way. The only way
out I see it is to bring ctime to an xattr, but that will need more iops
and gluster has to keep updating it on quite a few fops.
I did think about this when I was writing ec at the beginning. The idea
was that the point in time at which each fop is executed were controlled
by the client by adding an special xattr to each regular fop. Of course
this would require support inside the storage/posix xlator. At that
time, adding the needed support to other xlators seemed too complex for
me, so I decided to do something similar to afr.
Anyway, the idea was like this: for example, when a write fop needs to
be sent, dht/afr/ec sets the current time in a special xattr, for
example 'glusterfs.time'. It can be done in a way that if the time is
already set by a higher xlator, it's not modified. This way DHT could
set the time in fops involving multiple afr subvolumes. For other fops,
would be afr who sets the time. It could also be set directly by the top
most xlator (fuse), but that time could be incorrect because lower
xlators could delay the fop execution and reorder it. This would need
more thinking.
That xattr will be received by storage/posix. This xlator will determine
what times need to be modified and will change them. In the case of a
write, it can decide to modify mtime and, maybe, atime. For a mkdir or
create, it will set the times of the new file/directory and also the
mtime of the parent directory. It depends on the specific fop being
processed.
mtime, atime and ctime (or even others) could be saved in a special
posix xattr instead of relying on the file system attributes that cannot
be modified (at least for ctime).
This solution doesn't require extra fops, So it seems quite clean to me.
The additional I/O needed in posix could be minimized by implementing a
metadata cache in storage/posix that would read all metadata on lookup
and update it on disk only at regular intervals and/or on invalidation.
All fops would read/write into the cache. This would even reduce the
number of I/O we are currently doing for each fop.
JOE: the idea of metadata cache is cool for read work loads, but for writes we
would end up doing double writes to the disk. i.e 1 for the actual write or 1 to update the setxattr.
IMHO we cannot have it in a write back cache (periodic flush to disk) and ctime/mtime/atime data loss
or inconsistency will be a problem. Your thoughts?
If we want to have all in physical storage at all times, gluster will be
slow. We only need to be posix compliant, and posix allows some degree
of "inconsistency" here. i.e. we are not forced to write to physical
storage until the user application sends a flush or similar request.
Note that there are xlators that currently take advantage of this: for
example write-behind and md-cache.
Almost all file systems (if not all) rely on this to improve
performance, otherwise they would be really slow.
Of course this could cause a temporal inconsistency between bricks, but
since all cluster xlators (dht, afr and ec) use special xattrs to track
consistency, a crash before flushing the metadata could be detected and
repaired (with additional care even a crash while flushing metadata
could be detected).
Xavi
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