Return-Path: To: Weiping Zhang Cc: axboe@kernel.dk, cohuck@redhat.com, mst@redhat.com, jasowang@redhat.com, linux-block@vger.kernel.org, virtualization@lists.linux-foundation.org Subject: Re: [RFC PATCH 0/2] use larger max_request_size for virtio_blk From: "Martin K. Petersen" References: Date: Thu, 05 Apr 2018 08:24:27 -0400 In-Reply-To: (Weiping Zhang's message of "Thu, 5 Apr 2018 18:09:58 +0800") Message-ID: MIME-Version: 1.0 Content-Type: text/plain List-ID: Weiping, > For virtio block device, actually there is no a hard limit for max > request size, and virtio_blk driver set -1 to > blk_queue_max_hw_sectors(q, -1U);. But it doesn't work, because there > is a default upper limitation BLK_DEF_MAX_SECTORS (1280 sectors). That's intentional (although it's an ongoing debate what the actual value should be). > So this series want to add a new helper > blk_queue_max_hw_sectors_no_limit to set a proper max reqeust size. BLK_DEF_MAX_SECTORS is a kernel default empirically chosen to strike a decent balance between I/O latency and bandwidth. It sets an upper bound for filesystem requests only. Regardless of the capabilities of the block device driver and underlying hardware. You can override the limit on a per-device basis via max_sectors_kb in sysfs. People generally do it via a udev rule. -- Martin K. Petersen Oracle Linux Engineering