[prev in list] [next in list] [prev in thread] [next in thread] 

List:       zfs-discuss
Subject:    Re: [zfs-discuss] Pulsing write performance
From:       Bob Friesenhahn <bfriesen () simple ! dallas ! tx ! us>
Date:       2009-08-29 21:04:29
Message-ID: Pine.SOC.4.64.0908291559040.1103 () freddy ! simplesystems ! org
[Download RAW message or body]

On Sat, 29 Aug 2009, David Bond wrote:
>
> Ok, so when the chache fills and needs to be flushed, when the flush 
> occurs it locks access to it, so no read? or writes can occur from 
> cache, and as everything will go through the arc, nothing can happen 
> until the arc has finished its flush.

It has not been proven that reads from the ARC stop.  It is clear that 
reads from physical disk temporarily stop.  It is not clear (to me) if 
reads from physical disk stop because of the huge number of TXG sync 
write operations (up to 5 seconds worth) which are queued prior to the 
read request, or if reads are intentionally blocked due to some sort 
of coherency management.

> And to compensate for this, I would have to either reduce the cache 
> size to one that is small enough that the disk array can write it at 
> such a speed that the pauses are reduced to ones that are not really 
> noticable.

That would work.  There is likely to be more total physical I/O though 
since delaying the writes tends to eliminate many redundant writes. 
For example, an application which re-writes the same file over and 
over again would be sending more of that data to physical disk.

Bob
--
Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
[prev in list] [next in list] [prev in thread] [next in thread]