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Re: [Linux-cluster] AW: GNBD multipath with devicemapper? -- possible solution



Hi,

thank you for yor reply.

Benjamin Marzinski wrote:

On Tue, Apr 19, 2005 at 08:37:54AM +0200, Hansjoerg Maurer wrote:


The most anoying point is for me at the moment the differnence between gnbd read and write performance.
Therefore I am glad, that you as a gnbd-developer answered...
In my tests, gnbd write is about two to three times faster the gnbd reads.
I tried a lot of things (exporting cached, changing readahead with blockdev command (on the underlying device), changing TCP-IP buffersizes)
but I had nor improvement.


In the upper example, I get a write speed of about 85MB/s over gnbd and a read speed of about 26 MB/s .
(the underlying device's sda and sdb manages about 50MB/s (read and write).
Therefore read speed is very good....



Um. On my machines I get 41.6707 MB/sec for reads and 41.6805 MB/sec for writes using lmdd, so I don't see your difference. If I just write to the gnbd device, and don't sync the device after the writes I get a write speed of 54.3400 MB/sec. This only takes into account how fast gnbd can write out all the requests, not how long it takes for them to actually get written to disk on the server. I would be suspicious of a gnbd write speed that is higher than the served device's speed.

-Ben

the device speed of one disk is about 50 MB/s, but the two gnbd's (zwo seperate disks) are bundeled to one striped lvm.
Therefore the speed is possible.
I have seen the difference in read and write speed on two installations.
Kernel 2.6.9-6.38.EL and gfs from RHEL4 cvs , tested with cfq and anticipatory elevator.
What kernel are you using?


Thank you very much

Hansjörg







--
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Dr.  Hansjoerg Maurer           | LAN- & System-Manager
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