Re: [Qemu-devel] [PATCH v3] block/rbd: add preallocation support

[Jason Dillaman]

On Mon, Jul 29, 2019 at 5:40 AM Stefano Garzarella <address@hidden> wrote:
>
> On Fri, Jul 26, 2019 at 08:46:56AM -0400, Jason Dillaman wrote:
> > On Fri, Jul 26, 2019 at 4:48 AM Stefano Garzarella <address@hidden> wrote:
> > >
> > > On Thu, Jul 25, 2019 at 09:30:30AM -0400, Jason Dillaman wrote:
> > > > On Thu, Jul 25, 2019 at 4:13 AM Stefano Garzarella <address@hidden> 
> > > > 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 <address@hidden> 
> > > > > > 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 <address@hidden>
> > > > > > > ---
> > > > > > > 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, &current_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
> > >
> > > My idea was to allocate a small buffer for rbd_writesame and use the
> > > same to write the remaining bytes that should be a few.
> > > If the buffer was not allocated (so we didn't use the rbd_writesame),
> > > I'll allocate the big one:
> > >
> > >         if (ws_zero_supported) {
> > >             buf_size = 512;
> > >             buf = g_malloc0(buf_size);
> > >             ...
> > >         }
> > >
> > >         if (!buf) {
> > >             buf_size = object_size * max_concurrent_ops;
> > >             buf = g_malloc0(buf_size);
> > >         }
> > >
> > >         while (current_offset < offset) {
> > >             bytes = rbd_write(...)
> > >             ...
> > >         }
> > >
> > > > 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
> > >
> > > Why "object size * max concurrent ops" and not
> > > "stripe_unit * max concurrent ops"?
> > > IIUC stripe_unit can be smaller than object size.
> >
> > Correct, stripe unit *must* be smaller than the object size (and both
> > are powers of two). However, it's more efficient to send fewer larger
> > writes to a backing object than sending more small writes --
> > especially in the case of writesame where you don't have the network
> > overhead of transferring a large zeroed buffer. Replacing the full
> > backing object is even more efficient since it will just need to
> > perform a single backing disk allocation that will be continuous
> > instead of fragmented.
> >
>
> Okay, so IIUC I should do the following:
> - if we can use rbd_writesame
>     ~ allocates a buffer of 512 bytes
>     ~ writes "object size * max concurrent ops" bytes per call
> - if we use rbd_write
>     ~ allocates a buffer of "object size * max concurrent ops" bytes
>     ~ writes the entire buffer per call (aligning to the stripe unit)
>
> Is that correct?

Yes, that is correct.


> Thanks,
> Stefano


-- 
Jason