Re: [NFS] NFS Digest, Vol 18, Issue 70 (NFS performance problems)

["J. Bruce Fields" <bfields@fieldses.org>]

On Mon, Dec 03, 2007 at 04:49:42PM -0500, Wendy Cheng wrote:
> J. Bruce Fields wrote:
>> On Mon, Dec 03, 2007 at 04:13:02PM -0500, Wendy Cheng wrote:
>>   
>>> Or use cluster (a backup server is quite affordable nowadays) ? Was about 
>>> to kick off a new discussion about this ...
>>>
>>> I did a prototype about 4 years ago on 2.4 kernel where the RPC reply 
>>> cache (slightly modified to include raw NFS request packets) was mirrored 
>>> by backup server (in memory). The reply was delayed to go back to client 
>>> until the mirrored reply cache entry was acknowledged by the backup 
>>> server. Upon crash, the backup server piggybacked its logic on ext3's 
>>> journal recovery code. For reply cache entries not replayed or not 
>>> recognized by jbd, nfsd resent the NFS raw requests down to filesystem 
>>> just like any new arrived requested. The prototype code was able to gain 
>>> at least 70% of the async mode performance without losing the data.
>>>
>>> One of other issues with our current linux-based NFS cluster failover is 
>>> also right in this arena - that is, upon failover, the non-idempotent 
>>> could introduce stale filehandle errors that have been causing headaches 
>>> with some of the applications.
>>>     
>>
>> How exactly do the stale filehandles happen?
>>   
>
> Unless someone has fixed it .. last time we looked .. one of the causes was 
> like this:
>
> A "delete" was successfully executed on one server but before replying to 
> client, failover occurred. The retransmitted request was sent to take-over 
> server that subsequently couldn't find the file (since the file had gone). 
> A stale filehandle (or maybe an EACCESS or EPERM, forgot the details 
> though) was returned.

OK, makes sense.  But the REMOVE operation takes a filehandle (for a
parent) and a name (the name of the thing to remove), so if it's already
been removed you'd expect something like ENOENT.

--b.

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