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Re: [PATCH] block/blkio: Tolerate device size changes
From: |
Kevin Wolf |
Subject: |
Re: [PATCH] block/blkio: Tolerate device size changes |
Date: |
Wed, 2 Nov 2022 12:56:22 +0100 |
Am 29.10.2022 um 14:20 hat Alberto Faria geschrieben:
> Some libblkio drivers may be able to work with regular files (e.g.,
> io_uring) or otherwise resizable devices. Conservatively set
> BlockDriver::has_variable_length to true to ensure bdrv_nb_sectors()
> always gives up-to-date results.
>
> Also implement BlockDriver::bdrv_co_truncate for the case where no
> preallocation is needed and the device already has a size compatible
> with what was requested.
>
> Signed-off-by: Alberto Faria <afaria@redhat.com>
> ---
>
> This is based on Stefan's block tree:
> https://gitlab.com/stefanha/qemu/-/commits/block
>
> block/blkio.c | 27 +++++++++++++++++++++++++++
> 1 file changed, 27 insertions(+)
>
> diff --git a/block/blkio.c b/block/blkio.c
> index 82f26eedd2..190454cdbe 100644
> --- a/block/blkio.c
> +++ b/block/blkio.c
> @@ -845,6 +845,31 @@ static int64_t blkio_getlength(BlockDriverState *bs)
> return capacity;
> }
>
> +static int coroutine_fn blkio_truncate(BlockDriverState *bs, int64_t offset,
> + bool exact, PreallocMode prealloc,
> + BdrvRequestFlags flags, Error **errp)
> +{
> + int64_t current_length;
> +
> + if (prealloc != PREALLOC_MODE_OFF) {
> + error_setg(errp, "Unsupported preallocation mode '%s'",
> + PreallocMode_str(prealloc));
> + return -ENOTSUP;
> + }
> +
> + current_length = blkio_getlength(bs);
> +
> + if (offset > current_length) {
> + error_setg(errp, "Cannot grow device");
> + return -EINVAL;
> + } else if (exact && offset != current_length) {
> + error_setg(errp, "Cannot resize device");
> + return -ENOTSUP;
> + }
> +
> + return 0;
> +}
> +
> static int blkio_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
> {
> return 0;
> @@ -960,10 +985,12 @@ static void blkio_refresh_limits(BlockDriverState *bs,
> Error **errp)
> { \
> .format_name = name, \
> .protocol_name = name, \
> + .has_variable_length = true, \
I don't think this is a good idea, it will read the capacity from
libblkio before every single request, which can't result in good
performance.
We're generally only using .has_variable_length for removable media on
the backend (i.e. for host_cdrom and filters that can potentially sit on
top of it, but will used the cached value on the protocol level again
for other protocol drivers).
If you resize a host_device, you need to use QMP 'block_resize'
explicitly. I think it should be the same for libblkio.
Kevin
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