Speedy Tapes

Came across an odd problem last week. We had an LT03 tape connected to a T2000 via a qlogic card. We could only write about 50MB/s to the thing, should have been getting around 2Gb/s (bit, not byte). Everything was connected at 2Gb/s. Odd. Dtracing showed that the biowait() was taking a really long time. Seems the qlc driver was taking its time writing. Reading from the tape was fine, no worries there, could get 250MB/s there.

Then late Friday evening (GMT time) one of the guys who works in storage and knows about the qlogic driver pointed out this setting in the qlc.conf (for s10updates mind):

pci-max-read-request
He asked that I try with this set to 2048.

Ok then.

The throughput went from 50MB/s to 250MB/s just like that :)
So if you've got qlogic fibre to tape and want faster throughput try this out.

pci-max-read-request=2048;
It was a slow tape, but now no longer!
Comments:

Can somebody explain more about what that parameter is doing? Thanks

Posted by johnhl on August 29, 2006 at 06:11 PM IST #

okay, looked around and got the ideas and want to confirm it works this way, on pci config space at offset 44 (PCI-X Command/Next Item) bit [19:18] MXR register. Max Memory Read Byte Count. This register is intened to be modified by system software to tune system performance. Not sure why qlogic's driver modify it.

Posted by johnhl on August 29, 2006 at 06:48 PM IST #

Does it work only with Qlogic FC cards or with Qlogic SCSI cards as well ? przemol

Posted by przemol on August 30, 2006 at 07:39 AM IST #

Przemol: tis only for the qlc driver which is fibre.

Posted by smg on August 30, 2006 at 08:58 AM IST #

Johnhl ! Your right about wondering why the qlogic driver is doing the setting and not the system. Its deeper than that. The OBP is what should be looking after this. Theres a bug logged already on this and is fast being fixed.

Posted by smg on September 01, 2006 at 03:31 AM IST #

Post a Comment:
Comments are closed for this entry.
About

smg

Search

Archives
« May 2015
MonTueWedThuFriSatSun
    
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
       
Today