On Thu, Jul 25, 2019 at 4:13 AM Stefano Garzarella <sgarz...@redhat.com> wrote:
>
> On Wed, Jul 24, 2019 at 01:48:42PM -0400, Jason Dillaman wrote:
> > On Tue, Jul 23, 2019 at 3:13 AM Stefano Garzarella <sgarz...@redhat.com>
> > wrote:
> > >
> > > This patch adds the support of preallocation (off/full) for the RBD
> > > block driver.
> > > If rbd_writesame() is available and supports zeroed buffers, we use
> > > it to quickly fill the image when full preallocation is required.
> > >
> > > Signed-off-by: Stefano Garzarella <sgarz...@redhat.com>
> > > ---
> > > v3:
> > > - rebased on master
> > > - filled with zeroed buffer [Max]
> > > - used rbd_writesame() only when we can disable the discard of zeroed
> > > buffers
> > > - added 'since: 4.2' in qapi/block-core.json [Max]
> > > - used buffer as large as the "stripe unit"
> > > ---
> > > block/rbd.c | 202 ++++++++++++++++++++++++++++++++++++++++---
> > > qapi/block-core.json | 5 +-
> > > 2 files changed, 192 insertions(+), 15 deletions(-)
> > >
> > > diff --git a/block/rbd.c b/block/rbd.c
> > > index 59757b3120..d923a5a26c 100644
> > > --- a/block/rbd.c
> > > +++ b/block/rbd.c
> > > @@ -64,6 +64,7 @@
> > > #define OBJ_MAX_SIZE (1UL << OBJ_DEFAULT_OBJ_ORDER)
> > >
> > > #define RBD_MAX_SNAPS 100
> > > +#define RBD_DEFAULT_CONCURRENT_OPS 10
> > >
> > > /* The LIBRBD_SUPPORTS_IOVEC is defined in librbd.h */
> > > #ifdef LIBRBD_SUPPORTS_IOVEC
> > > @@ -104,6 +105,7 @@ typedef struct BDRVRBDState {
> > > char *image_name;
> > > char *snap;
> > > uint64_t image_size;
> > > + bool ws_zero_supported; /* rbd_writesame() supports zeroed buffers */
> > > } BDRVRBDState;
> > >
> > > static int qemu_rbd_connect(rados_t *cluster, rados_ioctx_t *io_ctx,
> > > @@ -333,6 +335,155 @@ static void qemu_rbd_memset(RADOSCB *rcb, int64_t
> > > offs)
> > > }
> > > }
> > >
> > > +static int qemu_rbd_get_max_concurrent_ops(rados_t cluster)
> > > +{
> > > + char buf[16];
> > > + int ret, max_concurrent_ops;
> > > +
> > > + ret = rados_conf_get(cluster, "rbd_concurrent_management_ops", buf,
> > > + sizeof(buf));
> > > + if (ret < 0) {
> > > + return RBD_DEFAULT_CONCURRENT_OPS;
> > > + }
> > > +
> > > + ret = qemu_strtoi(buf, NULL, 10, &max_concurrent_ops);
> > > + if (ret < 0) {
> > > + return RBD_DEFAULT_CONCURRENT_OPS;
> > > + }
> > > +
> > > + return max_concurrent_ops;
> > > +}
> > > +
> > > +static int qemu_rbd_do_truncate(rados_t cluster, rbd_image_t image,
> > > + int64_t offset, PreallocMode prealloc,
> > > + bool ws_zero_supported, Error **errp)
> > > +{
> > > + uint64_t current_length;
> > > + char *buf = NULL;
> > > + int ret;
> > > +
> > > + ret = rbd_get_size(image, ¤t_length);
> > > + if (ret < 0) {
> > > + error_setg_errno(errp, -ret, "Failed to get file length");
> > > + goto out;
> > > + }
> > > +
> > > + if (current_length > offset && prealloc != PREALLOC_MODE_OFF) {
> > > + error_setg(errp, "Cannot use preallocation for shrinking files");
> > > + ret = -ENOTSUP;
> > > + goto out;
> > > + }
> > > +
> > > + switch (prealloc) {
> > > + case PREALLOC_MODE_FULL: {
> > > + uint64_t buf_size, current_offset = current_length;
> > > + ssize_t bytes;
> > > +
> > > + ret = rbd_get_stripe_unit(image, &buf_size);
> > > + if (ret < 0) {
> > > + error_setg_errno(errp, -ret, "Failed to get stripe unit");
> > > + goto out;
> > > + }
> > > +
> > > + ret = rbd_resize(image, offset);
> > > + if (ret < 0) {
> > > + error_setg_errno(errp, -ret, "Failed to resize file");
> > > + goto out;
> > > + }
> > > +
> > > + buf = g_malloc0(buf_size);
> > > +
> > > +#ifdef LIBRBD_SUPPORTS_WRITESAME
> > > + if (ws_zero_supported) {
> > > + uint64_t writesame_max_size;
> > > + int max_concurrent_ops;
> > > +
> > > + max_concurrent_ops =
> > > qemu_rbd_get_max_concurrent_ops(cluster);
> > > + /*
> > > + * We limit the rbd_writesame() size to avoid to spawn more
> > > then
> > > + * 'rbd_concurrent_management_ops' concurrent operations.
> > > + */
> > > + writesame_max_size = MIN(buf_size * max_concurrent_ops,
> > > INT_MAX);
> >
> > In the most efficient world, the 'buf_size' would be some small, fixed
> > power of 2 value (like 512 bytes) since there isn't much need to send
> > extra zeroes. You would then want to writesame the full stripe period
> > (if possible), where a stripe period is the data block object size
> > (defaults to 4MiB and is availble via 'rbd_stat') * the stripe count.
> > In this case, the stripe count becomes the number of in-flight IOs.
> > Therefore, you could substitute its value w/ the max_concurrent_ops to
> > ensure you are issuing exactly max_concurrent_ops IOs per
> > rbd_writesame call.
> >
>
> Initially, I had a fixed buffer size to 4 KiB, but I noted that, when
> we didn't use writesame, the rbd_write() was very slow, so I used the
> stripe unit as a buffer size.
>
> Do you think is better to have a small buffer (512 byte) when we use
> writesame or a 'stripe unit' buffer when we can't use it?
I'd use a small buffer for rbd_writesame and then just reallocate the
buffer to a larger size for "rbd_write". It would be most efficient to
allocate a "object size * max concurrent ops" -sized buffer (up to
some reasonable maximum) for the standard rbd_write. Just make sure
your "rbd_writes" offsets / length is aligned to the stripe period for
the most efficient IO (i.e. the initial write might be smaller than
the stripe period if the starting offset is unaligned).
> > > +
> > > + while (offset - current_offset > buf_size) {
> > > + bytes = MIN(offset - current_offset, writesame_max_size);
> > > + /*
> > > + * rbd_writesame() supports only request where the size
> > > of the
> > > + * operation is multiple of buffer size.
> > > + */
> > > + bytes -= bytes % buf_size;
> > > +
> > > + bytes = rbd_writesame(image, current_offset, bytes, buf,
> > > + buf_size, 0);
> >
> > If the RBD in-memory cache is enabled during this operation, the
> > writesame will effectively just be turned into a write. Therefore,
> > when pre-allocating, you will want to disable the cache.
> >
>
> During the creation, when preallocation is often used, we disable the cache:
>
> static int qemu_rbd_do_create(BlockdevCreateOptions *options,
> const char *keypairs, const char
> *password_secret,
> Error **errp)
> {
> ...
>
> ret = qemu_rbd_connect(&cluster, &io_ctx, opts->location, false, keypairs,
> ^^ cache param
> ...
> }
>
>
> Do you think I should disable it in any case during the preallocation?
Assuming you can grow an image w/ full preallocation, I would
definitely want to ensure that the cache is disabled since otherwise
you would just be performing regular (non-offloaded) writes.
>
>
> > > + if (bytes < 0) {
> > > + ret = bytes;
> > > + error_setg_errno(errp, -ret,
> > > + "Failed to write for
> > > preallocation");
> > > + goto out;
> > > + }
> > > +
> > > + current_offset += bytes;
> > > + }
> > > + }
> > > +#endif /* LIBRBD_SUPPORTS_WRITESAME */
> > > +
> > > + while (current_offset < offset) {
> > > + bytes = rbd_write(image, current_offset,
> > > + MIN(offset - current_offset, buf_size),
> > > buf);
> > > + if (bytes < 0) {
> > > + ret = bytes;
> > > + error_setg_errno(errp, -ret,
> > > + "Failed to write for preallocation");
> > > + goto out;
> > > + }
> > > +
> > > + current_offset += bytes;
> > > + }
> > > +
> > > + ret = rbd_flush(image);
> > > + if (ret < 0) {
> > > + error_setg_errno(errp, -ret, "Failed to flush the file");
> > > + goto out;
> > > + }
> > > +
> > > + break;
> > > + }
> > > + case PREALLOC_MODE_OFF:
> > > + ret = rbd_resize(image, offset);
> >
> > I'm not familiar enough w/ the QEMU block code, but why would the
> > PREALLOC_MODE_FULL case not need to resize the image?
>
> PREALLOC_MODE_FULL need too, I did it just before the g_malloc0() in
> this patch :-)
Sorry I missed it. Would it make more sense to just do it before the
switch statement so that you don't duplicate the code and resulting
error handling? I guess just validate that the prealloc mode is
supported before issuing the resize.
> Thanks,
> Stefano
--
Jason
