On Wed, 2019-02-27 at 12:52 -0500, David Noveck wrote:
However note that the counter argument to what you state above is that _if_ the server requires a layoutcommit before it will acknowledge a file size change, then pNFS is likely to underperform for applications such as databases or VMs where each record is required to be written in stable mode. IOW: If all writes that need to be stable are also required to be acknowledged with a layoutcommit (to the MDS),
But it is not true that *all* writes that need to be stable are also required to be acknowledged with a layoutcommit (to the MDS. Only those that potentially change the file size require this.
That's true for POSIX O_DSYNC writes, but it is not true for O_SYNC. In the latter case, the timestamps are required to be updated synchronously as well, which implies a layoutcommit.
then your ability to scale out your server will be in doubt
For many applications, particularly databases, it will easy to make sure that the writes that potentially change the file size are few and far between.
If the database uses O_DSYNC, yes.
On Tue, Feb 26, 2019 at 8:12 PM Trond Myklebust < trondmy@hammerspace.com> wrote:
Trond Myklebust wrote: [stuff snipped]
Please see the Errata ID 2751 http://www.rfc-editor.org/errata/eid2751
I'll admit I hadn't seen this errata before. However, it seems to be specific to the File Layout. For the Flexible File Layout...
When I look in RFC-8435, I cannot find anything that states that a LayoutCommit is only required for case(s) where a Commit to the Storage Server is required. Sec. 2.1 Clearly states that a Commit to the Storage Server is required before the client does a LayoutCommit when the write(s) were not done FILE_SYNC. However, I do not see any indication that the LayoutCommit is not to be done for the case where the write(s) are done FILE_SYNC.
FF_FLAGS_NO_LAYOUTCOMMIT can be used to indicate to a client that LayoutCommits are not required, but this does not be dependent on how the write(s) to the Storage Server were done.
The only way a Flexible File layout Metadata server can know what
On Wed, 2019-02-27 at 00:13 +0000, Rick Macklem wrote: the
current file size is (when a read/write layout is issued to a client) is to do a Getattr to the Storage Server. If a client is not required to do a LayoutCommit when the write(s) to the Storage Server are done FILE_SYNC, then the Metadata server must do Getattr RPCs to the Storage Server whenever it needs an up to date file size if a read/write layout is issued to a client.
This can result in a lot of overhead that can be avoided by requiring the LayoutCommit to be done by a client after writing to a Storage Server, irrespective of the need for a Commit to the Storage Server. As such, I would rather not have this errata applied to RFC-8435.
Fair enough. I agree that the errata in question only applies to the pNFS files layout, however you were talking about RFC5661 and whether or not we were interpreting that correctly. Since RFC5661 only refers to about the behaviour of the pNFS files layout, then I assumed that was what you were referring to.
For flexfiles we may have a bug in the layoutcommit case. However note that the counter argument to what you state above is that _if_ the server requires a layoutcommit before it will acknowledge a file size change, then pNFS is likely to underperform for applications such as databases or VMs where each record is required to be written in stable mode. IOW: If all writes that need to be stable are also required to be acknowledged with a layoutcommit (to the MDS), then your ability to scale out your server will be in doubt.
-- Trond Myklebust Linux NFS client maintainer, Hammerspace trond.myklebust@hammerspace.com