Re: [Qemu-devel] [PATCH] block: gluster: Probe alignment limits

[Nir Soffer <nsoffer@redhat.com>]

On Sun, Aug 18, 2019 at 12:21 AM Nir Soffer <nirsof@gmail.com> wrote:

> Implement alignment probing similar to file-posix, by reading from the
> first 4k of the image.
>
> Before this change, provisioning a VM on storage with sector size of
> 4096 bytes would fail when the installer try to create filesystems. Here
> is an example command that reproduces this issue:
>
>     $ qemu-system-x86_64 -accel kvm -m 2048 -smp 2 \
>         -drive
> file=gluster://gluster1/gv0/fedora29.raw,format=raw,cache=none \
>         -cdrom Fedora-Server-dvd-x86_64-29-1.2.iso
>
> The installer fails in few seconds when trying to create filesystem on
> /dev/mapper/fedora-root. In error report we can see that it failed with
> EINVAL (I could not extract the error from guest).
>
> Copying disk fails with EINVAL:
>
>     $ qemu-img convert -p -f raw -O raw -t none -T none \
>         gluster://gluster1/gv0/fedora29.raw \
>         gluster://gluster1/gv0/fedora29-clone.raw
>     qemu-img: error while writing sector 4190208: Invalid argument
>
> This is a fix to same issue fixed in commit a6b257a08e3d (file-posix:
> Handle undetectable alignment) for gluster:// images.
>
> This fix has the same limit, that the first block of the image should be
> allocated, otherwise we cannot detect the alignment and fallback to a
> safe value (4096) even when using storage with sector size of 512 bytes.
>
> Signed-off-by: Nir Soffer <nsoffer@redhat.com>
> ---
>  block/gluster.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 47 insertions(+)
>
> diff --git a/block/gluster.c b/block/gluster.c
> index f64dc5b01e..d936240b72 100644
> --- a/block/gluster.c
> +++ b/block/gluster.c
> @@ -52,6 +52,9 @@
>
>  #define GERR_INDEX_HINT "hint: check in 'server' array index '%d'\n"
>
> +/* The value is known only on the server side. */
> +#define MAX_ALIGN 4096
> +
>  typedef struct GlusterAIOCB {
>      int64_t size;
>      int ret;
> @@ -902,8 +905,52 @@ out:
>      return ret;
>  }
>
> +/*
> + * Check if read is allowed with given memory buffer and length.
> + *
> + * This function is used to check O_DIRECT request alignment.
> + */
> +static bool gluster_is_io_aligned(struct glfs_fd *fd, void *buf, size_t
> len)
> +{
> +    ssize_t ret = glfs_pread(fd, buf, len, 0, 0, NULL);
> +    return ret >= 0 || errno != EINVAL;
> +}
> +
> +static void gluster_probe_alignment(BlockDriverState *bs, struct glfs_fd
> *fd,
> +                                    Error **errp)
> +{
> +    char *buf;
> +    size_t alignments[] = {1, 512, 1024, 2048, 4096};
> +    size_t align;
> +    int i;
> +
> +    buf = qemu_memalign(MAX_ALIGN, MAX_ALIGN);
> +
> +    for (i = 0; i < ARRAY_SIZE(alignments); i++) {
> +        align = alignments[i];
> +        if (gluster_is_io_aligned(fd, buf, align)) {
> +            /* Fallback to safe value. */
> +            bs->bl.request_alignment = (align != 1) ? align : MAX_ALIGN;
> +            break;
> +        }
> +    }
> +
> +    qemu_vfree(buf);
> +
> +    if (!bs->bl.request_alignment) {
> +        error_setg(errp, "Could not find working O_DIRECT alignment");
> +        error_append_hint(errp, "Try cache.direct=off\n");
> +    }
> +}
> +
>  static void qemu_gluster_refresh_limits(BlockDriverState *bs, Error
> **errp)
>  {
> +    BDRVGlusterState *s = bs->opaque;
> +
> +    gluster_probe_alignment(bs, s->fd, errp);
> +
> +    bs->bl.min_mem_alignment = bs->bl.request_alignment;
> +    bs->bl.opt_mem_alignment = MAX(bs->bl.request_alignment, MAX_ALIGN);
>      bs->bl.max_transfer = GLUSTER_MAX_TRANSFER;
>  }
>
> --
> 2.20.1
>
>
To debug this I added this temporary patch:

diff --git a/block/gluster.c b/block/gluster.c
index d2d187490b..790ef4251b 100644
--- a/block/gluster.c
+++ b/block/gluster.c
@@ -912,6 +912,7 @@ out:
 static bool gluster_is_io_aligned(struct glfs_fd *fd, void *buf, size_t
len)
 {
     ssize_t ret = glfs_pread(fd, buf, len, 0, 0, NULL);
+    printf("gluster_is_io_aligned len=%ld ret=%ld errno=%d\n", len, ret,
errno);
     return ret >= 0 || errno != EINVAL;
 }

@@ -940,6 +941,9 @@ static void gluster_probe_alignment(BlockDriverState
*bs, struct glfs_fd *fd,
         error_setg(errp, "Could not find working O_DIRECT alignment");
         error_append_hint(errp, "Try cache.direct=off\n");
     }
+
+    printf("Probed aligment for %s request_alignment=%d\n",
+           bs->filename, bs->bl.request_alignment);
 }

 static void qemu_gluster_refresh_limits(BlockDriverState *bs, Error **errp)

Here is example run with volume with sector size of 512 bytes:

$ sudo mount -t glusterfs gluster1:/gv1 /tmp/gv1
$ dd if=/dev/zero bs=1M count=100 | tr "\0" "x" > /tmp/gv1/src.raw
$ truncate -s 100m /tmp/gv1/dst.raw
$ dd if=/dev/zero bs=1 count=1 of=/tmp/gv1/dst.raw conv=notrunc

$ ./qemu-img convert -n -f raw -O raw -t none -T none
gluster://gluster1/gv1/src.raw gluster://gluster1/gv1/dst.raw
gluster_is_io_aligned len=1 ret=-1 errno=22
gluster_is_io_aligned len=512 ret=512 errno=0
Probed aligment for gluster://gluster1/gv1/src.raw request_alignment=512
gluster_is_io_aligned len=1 ret=-1 errno=22
gluster_is_io_aligned len=512 ret=512 errno=0
Probed aligment for gluster://gluster1/gv1/dst.raw request_alignment=512

And with volume with sector size of 4096 bytes:

$ sudo mount -t glusterfs gluster1:/gv0 /tmp/gv0
$ dd if=/dev/zero bs=1M count=100 | tr "\0" "x" > /tmp/gv0/src.raw
$ truncate -s 100m /tmp/gv0/dst.raw
$ dd if=/dev/zero bs=1 count=1 of=/tmp/gv0/dst.raw conv=notrunc

$ ./qemu-img convert -n -f raw -O raw -t none -T none
gluster://gluster1/gv0/src.raw gluster://gluster1/gv0/dst.raw
gluster_is_io_aligned len=1 ret=-1 errno=22
gluster_is_io_aligned len=512 ret=-1 errno=22
gluster_is_io_aligned len=1024 ret=-1 errno=22
gluster_is_io_aligned len=2048 ret=-1 errno=22
gluster_is_io_aligned len=4096 ret=4096 errno=0
Probed aligment for gluster://gluster1/gv0/src.raw request_alignment=4096
gluster_is_io_aligned len=1 ret=-1 errno=22
gluster_is_io_aligned len=512 ret=-1 errno=22
gluster_is_io_aligned len=1024 ret=-1 errno=22
gluster_is_io_aligned len=2048 ret=-1 errno=22
gluster_is_io_aligned len=4096 ret=4096 errno=0
Probed aligment for gluster://gluster1/gv0/dst.raw request_alignment=4096