[DRBD-user] Large block IO bottleneck

Leroy van Logchem leroy.vanlogchem at wldelft.nl
Wed Jan 3 14:02:50 CET 2007


>> Hi there I am using DRBD 0.7.21 with iSCSI Enterprise Target 0.4.14 on
>> CentOS 4.4.
>>
>> When I run iSCSI direct to the LVM lv on top of hardware RAID I can get
>> 225 MB/s over two sessions in MPIO with 256K block size, but when I put
>> DRBD in-between iSCSI and LVM the throughput tops out at 80 MB/s and I
>> can't seem to go over that.
>>
>> DRBD seems to report it's max number of sectors as 8 (4K), does that
>> mean each io operation is limited to 4K? My hardware raid reports it's
>> max sectors as 128, could this explain the reduction to 1/3 throughput?
>>     
>
> Hi,
>
> The cause for the limitation to 4k is the Linux-2.4 compabitility of 
> DRBD-0.7.
>
> Repeat your test with drbd-8.0(rc1).
>
> drbd-8.0 will do BIOs up to 32k, but much more important are other
> changes (e.g. the non blocking make_request() function), that makes
> drbd-8.0 to scale much better with high end hardware.
>   
I guess many 0.7.x users could benefit from the 32k BIO's. Is it trivial 
to patch 0.7.x?
( Without breaking 2.4 support using a 'configure --with-large-bio' )



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