From 07dc43478f4d96eb1cd21a40a26262beb1ca2dd6 Mon Sep 17 00:00:00 2001 From: Maxim Levitsky Date: Mon, 27 Apr 2020 08:39:13 +0200 Subject: [PATCH] file-posix: Use max transfer length/segment count only for SCSI passthrough RH-Author: Maxim Levitsky Message-id: <20200420155959.20876-2-mlevitsk@redhat.com> Patchwork-id: 95748 O-Subject: [RHEL-7.9 qemu-kvm-rhev PATCH 1/1] file-posix: Use max transfer length/segment count only for SCSI passthrough Bugzilla: 1819253 RH-Acked-by: Stefan Hajnoczi RH-Acked-by: Stefano Garzarella RH-Acked-by: Max Reitz Regular kernel block devices (/dev/sda*, /dev/nvme*, etc) don't have max segment size/max segment count hardware requirements exposed to the userspace, but rather the kernel block layer takes care to split the incoming requests that violate these requirements. Allowing the kernel to do the splitting allows qemu to avoid various overheads that arise otherwise from this. This is especially visible in nbd server, exposing as a raw file, a mostly empty qcow2 image over the net. In this case most of the reads by the remote user won't even hit the underlying kernel block device, and therefore most of the overhead will be in the nbd traffic which increases significantly with lower max transfer size. In addition to that even for local block device access the peformance improves a bit due to less traffic between qemu and the kernel when large transfer sizes are used (e.g for image conversion) More info can be found at: https://bugzilla.redhat.com/show_bug.cgi?id=1647104 Signed-off-by: Maxim Levitsky Reviewed-by: Stefan Hajnoczi Reviewed-by: Eric Blake Reviewed-by: Pankaj Gupta Signed-off-by: Kevin Wolf (cherry picked from commit 867eccfed84f96b54f4a432c510a02c2ce03b430) Signed-off-by: Maxim Levitsky Signed-off-by: Miroslav Rezanina --- block/file-posix.c | 54 ++++++++++++++++++++++++++++-------------------------- 1 file changed, 28 insertions(+), 26 deletions(-) diff --git a/block/file-posix.c b/block/file-posix.c index 548424d..9e5cd68 100644 --- a/block/file-posix.c +++ b/block/file-posix.c @@ -1008,15 +1008,13 @@ static void raw_reopen_abort(BDRVReopenState *state) s->reopen_state = NULL; } -static int hdev_get_max_transfer_length(BlockDriverState *bs, int fd) +static int sg_get_max_transfer_length(int fd) { #ifdef BLKSECTGET int max_bytes = 0; - short max_sectors = 0; - if (bs->sg && ioctl(fd, BLKSECTGET, &max_bytes) == 0) { + + if (ioctl(fd, BLKSECTGET, &max_bytes) == 0) { return max_bytes; - } else if (!bs->sg && ioctl(fd, BLKSECTGET, &max_sectors) == 0) { - return max_sectors << BDRV_SECTOR_BITS; } else { return -errno; } @@ -1025,25 +1023,31 @@ static int hdev_get_max_transfer_length(BlockDriverState *bs, int fd) #endif } -static int hdev_get_max_segments(const struct stat *st) +static int sg_get_max_segments(int fd) { #ifdef CONFIG_LINUX char buf[32]; const char *end; - char *sysfspath; + char *sysfspath = NULL; int ret; - int fd = -1; + int sysfd = -1; long max_segments; + struct stat st; + + if (fstat(fd, &st)) { + ret = -errno; + goto out; + } sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments", - major(st->st_rdev), minor(st->st_rdev)); - fd = open(sysfspath, O_RDONLY); - if (fd == -1) { + major(st.st_rdev), minor(st.st_rdev)); + sysfd = open(sysfspath, O_RDONLY); + if (sysfd == -1) { ret = -errno; goto out; } do { - ret = read(fd, buf, sizeof(buf) - 1); + ret = read(sysfd, buf, sizeof(buf) - 1); } while (ret == -1 && errno == EINTR); if (ret < 0) { ret = -errno; @@ -1060,8 +1064,8 @@ static int hdev_get_max_segments(const struct stat *st) } out: - if (fd != -1) { - close(fd); + if (sysfd != -1) { + close(sysfd); } g_free(sysfspath); return ret; @@ -1073,19 +1077,17 @@ out: static void raw_refresh_limits(BlockDriverState *bs, Error **errp) { BDRVRawState *s = bs->opaque; - struct stat st; - if (!fstat(s->fd, &st)) { - if (S_ISBLK(st.st_mode) || S_ISCHR(st.st_mode)) { - int ret = hdev_get_max_transfer_length(bs, s->fd); - if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) { - bs->bl.max_transfer = pow2floor(ret); - } - ret = hdev_get_max_segments(&st); - if (ret > 0) { - bs->bl.max_transfer = MIN(bs->bl.max_transfer, - ret * getpagesize()); - } + if (bs->sg) { + int ret = sg_get_max_transfer_length(s->fd); + + if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) { + bs->bl.max_transfer = pow2floor(ret); + } + + ret = sg_get_max_segments(s->fd); + if (ret > 0) { + bs->bl.max_transfer = MIN(bs->bl.max_transfer, ret * getpagesize()); } } -- 1.8.3.1