On Tue, 17 Feb 2004 15:57:16, Miquel van Smoorenburg wrote: > For some reason, when using LVM, write requests get queued out > of order to the 3ware controller, which results in quite a bit > of seeking and thus performance loss. [..] > Okay I repeated some earlier tests, and I added some debug code in > several places. > > I added logging to tw_scsi_queue() in the 3ware driver to log the > start sector and length of each request. It logs something like: > 3wdbg: id 119, lba = 0x2330bc33, num_sectors = 256 > > With a perl script, I can check if the requests are sent to the > host in order. That outputs something like this: > > Consecutive: start 1180906348, length 7936 sec (3968 KB), requests: 31 > Consecutive: start 1180906340, length 8 sec (4 KB), requests: 1 > Consecutive: start 1180914292, length 7936 sec (3968 KB), requests: 31 > Consecutive: start 1180914284, length 8 sec (4 KB), requests: 1 > Consecutive: start 1180922236, length 7936 sec (3968 KB), requests: 31 > Consecutive: start 1180922228, length 8 sec (4 KB), requests: 1 > Consecutive: start 1180930180, length 7936 sec (3968 KB), requests: 31 > > See, 31 requests in order, then one request "backwards", then 31 in order, etc. I found out what causes this. It's get_request_wait(). When the request queue is full, and a new request needs to be created, __make_request() blocks in get_request_wait(). Another process wakes up first (pdflush / process submitting I/O itself / xfsdatad / etc) and sends the next bio's to __make_request(). In the mean time some free requests have become available, and the bios are merged into a new request. Those requests are submitted to the device. Then, get_request_wait() returns but the bio is not mergeable anymore - and that results in a backwards seek, severely limiting the I/O rate. Wouldn't it be better to allow the request allocation and queue the request, and /then/ put the process to sleep ? The queue will grow larger than nr_requests, but it does that anyway. Mike. _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/