Re: async vs. sync

[jehan procaccia <jehan.procaccia@int-evry.fr>]

Lever, Charles wrote:

>>This is what I expect in term of performances . I will continue my 
>>requests on the DEll/EMC hotline , but maybe the security of 
>>that AX100 
>>storage Processor (raid5, spare disk, double fiber attachement, UPS) 
>>allows me to use async export mode in such a case ?
>>    
>>
>
>the "async" export option changes the behavior of the NFS server
>daemons, not of the underlying local file system or storage subsystem.
>the problem is that changes made by clients will remain in your NFS
>server's memory and not get flushed onto permanent storage.
>
>so, i really don't think the storage subsystem will have any effect on
>the safety of your data before the data reaches permanent storage.  as
>someone else pointed out earlier, the solution is to use battery-backed
>main memory when using "async" (prestoserve for solaris?).
>
>as trond said, if your users and backup facilities can tolerate the loss
>of data during a crash, then it is perfectly fine to use "async."  most
>don't, however.
>
>btw, it is fairly well understood that RAID-5 and NFS servers don't mix
>well.  RAID-5's weakest point is that it doesn't handle small random
>writes very well, and that's exactly what is required of it when
>handling NFS traffic that consists mostly of metadata changes (file
>creates, deletes, and so on).  neil explained clearly how to make the
>best use of a RAID-5 with NFS: do your local file system journaling
>somewhere else.
>  
>
No, not yet, but  if it is safer and increase performances maybe I 
should do it !

Perhaps it's not the place to talk about ext3 here, but if someone on 
the list did already put their journal on a separate device, please 
confirm me those points:
 From what I read on man mkefs for ext3 FS I can create a journal on a 
separate FS :
mke2fs -O journal_dev external-journal
creates the journal FS, on which device ? -> internal scsi drive of my 
server or better placed on the dell/EMC SP ?

mke2fs -J device=/dev/external-journal /dev/emcpower
Format the FS and use the external journal just create above, but what is the recommended size of the 
external journal ?  when journal is internal it is said  the size of the journal must be at least 1024 filesystem blocks
(in my case blocks a 4K size) so journal is at least 4 Mb, but should it be bigger ?

Finally, can I "externalize" an already internal journal from production FS  (convert journal from inside to outside without reformating the FS ) ?

thanks.


>when trying your workload locally on the NFS server, realize that there
>are some optimizations that local file systems make, like caching and
>coalescing metadata updates, that the NFS protocol does not allow.  this
>affects especially workloads with lots of metadata change operations,
>because the NFS protocol requires each metadata update to reside on
>permanent storage before the NFS server replies to the client,
>effectively serializing the workload with storage activity.
>  
>



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