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

[Wendy Cheng <wcheng@redhat.com>]

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.

-- Wendy


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