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Message-ID: <47617B92.6020908@rtr.ca> Date: Thu, 13 Dec 2007 13:36:02 -0500 From: Mark Lord <lkml@....ca> To: Jens Axboe <axboe@...e.de>, IDE/ATA development list <linux-ide@...r.kernel.org>, Linux Kernel <linux-kernel@...r.kernel.org>, linux-scsi <linux-scsi@...r.kernel.org> Subject: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ? Jens, I'm experimenting here with trying to generate large I/O through libata, and not having much luck. The limit seems to be the number of hardware PRD (SG) entries permitted by the driver (libata:ata_piix), which is 128 by default. The problem is, the block layer *never* sends an SG entry larger than 8192 bytes, and even that size is exceptionally rare. Nearly all I/O segments are 4096 bytes, so I never see a single I/O larger than 512KB (128 * 4096). If I patch various parts of block and SCSI, this limit doesn't budge, but when I change the hardware PRD limit in libata, it scales by exactly whatever I set the new value to. This tells me that adjacent I/O segments are not being combined. I thought that QUEUE_FLAG_CLUSTER (aka. SCSI host .use_clustering=1) should result in adjacent single pages being combined into larger physical segments? This is x86-32 with latest 2.6.24-rc*. I'll re-test on older kernels next. ??? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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