[Gluster-users] AFR, writebehind, and debug/trace

[Shehjar Tikoo]

Barry Jaspan wrote:
> I just got started with glusterfs.  I read the docs over the weekend 
> and today created a simple setup: two servers exporting a brick and 
> one client mounting them with AFR. I am seeing very poor write 
> performance on a dd test, e.g.:
> 
> time dd if=/dev/zero of=./local-file bs=8192 count=125000
> 
> presumably due to a very large number of write operations (because 
> when I increase the blocksize to 64K, the performance increases by 
> 2x).  I enabled the writebehind translator but see no improvement.  I
>  then enabled a trace translator on both sides of the writebehind and
>  seem to be seeing that write-behind is not batching any of the 
> operations.
> 

By batching, if you mean aggregation of smaller requests to be sent as
one large request, then, no, write-behind does just that, it writes
  behind the actual write request. There is no write buffering going on
at this point. We've plans of incorporating write buffering in io-cache
for the 2.1 release.

-Shehjar


> Server vol file:
> 
> volume posix type storage/posix option directory 
> /mnt/glusterfsd-export end-volume volume locks type features/locks 
> subvolumes posix end-volume volume brick type performance/io-threads
>  option thread-count 8 subvolumes locks end-volume volume server type
>  protocol/server option transport-type tcp option 
> auth.addr.brick.allow * subvolumes brick end-volume
> 
> Client vol file:
> 
> volume remote1 type protocol/client option transport-type tcp option 
> remote-host web-1 option remote-subvolume brick end-volume volume 
> remote2 type protocol/client option transport-type tcp option 
> remote-host web-2 option remote-subvolume brick end-volume volume 
> replicate type cluster/replicate subvolumes remote1 remote2 
> end-volume volume trace-below type debug/trace subvolumes replicate 
> end-volume volume writebehind type performance/write-behind option 
> cache-size 1MB subvolumes trace-below end-volume volume trace-above 
> type debug/trace subvolumes writebehind end-volume
> 
> With this configuration,  I re-ran by dd test but with only 
> count=100. The log shows:
> 
> [root at web-3 glusterfs-mount]# grep trace 
> /var/log/glusterfs/mnt-glusterfs-mount.log | grep above | wc 245 3591
> 42117 [root at web-3 glusterfs-mount]# grep trace 
> /var/log/glusterfs/mnt-glusterfs-mount.log | grep below | wc 252 3678
> 43095
> 
> So, there are as many writes to trace-below as trace-above.
> 
> What am I not understanding?
> 
> Thanks!
> 
> Barry
> 
> 
> 
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