virtio_blk: set the zone write granularitygrafted

virtblk_read_zoned_limits() reads the write granularity that the device
reports in virtio_blk_zoned_characteristics and assigns it to the
physical block size and to io_min, but never to the limit that is named
after it. queue_limits.zone_write_granularity is left at zero, so
blk_validate_zoned_limits() raises it to the logical block size:

	if (lim->zone_write_granularity < lim->logical_block_size)
		lim->zone_write_granularity = lim->logical_block_size;

A device that reports a granularity coarser than its logical block size,
which is what the field exists to express, therefore has it silently
reduced. A 512e host managed disk passed through to a guest reports a
logical block size of 512 and a write granularity of 4096, and the guest
ends up with a zone write granularity of 512.

bio_split_alignment() returns lim->zone_write_granularity if it is non-zero
and bio_split_io_at() may split a bio with as per bio_split_alignment().
This can real to the write getting rejected by the host drive, as the write
is not aligned to the physical block size.

zonefs also takes its block size from bdev_zone_write_granularity(), so it
would incorrectly use 512 on a disk that requires 4096.

sd_zbc_read_zones() sets the limit from the physical block size for the
same reason. NVMe ZNS and null_blk leave it unset, but the fallback
gives the right answer for them, as their write granularity is the
logical block size. virtio carries a separate value that may exceed it.

Set the zone write granularity from the value that the device reports.

Fixes: 95bfec41bd3d ("virtio-blk: add support for zoned block devices")
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://patch.msgid.link/20260918140641.2031075-2-cassel@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 111 insertions, 0 deletions

diff --git a/Documentation/driver-api/md/raid5-cache.rst b/Documentation/driver-api/md/raid5-cache.rst
new file mode 100644
index 000000000..5f947cbc2
--- /dev/null
+++ b/Documentation/driver-api/md/raid5-cache.rst
@@ -0,0 +1,111 @@
+================
+RAID 4/5/6 cache
+================
+
+Raid 4/5/6 could include an extra disk for data cache besides normal RAID
+disks. The role of RAID disks isn't changed with the cache disk. The cache disk
+caches data to the RAID disks. The cache can be in write-through (supported
+since 4.4) or write-back mode (supported since 4.10). mdadm (supported since
+3.4) has a new option '--write-journal' to create array with cache. Please
+refer to mdadm manual for details. By default (RAID array starts), the cache is
+in write-through mode. A user can switch it to write-back mode by::
+
+	echo "write-back" > /sys/block/md0/md/journal_mode
+
+And switch it back to write-through mode by::
+
+	echo "write-through" > /sys/block/md0/md/journal_mode
+
+In both modes, all writes to the array will hit cache disk first. This means
+the cache disk must be fast and sustainable.
+
+write-through mode
+==================
+
+This mode mainly fixes the 'write hole' issue. For RAID 4/5/6 array, an unclean
+shutdown can cause data in some stripes to not be in consistent state, eg, data
+and parity don't match. The reason is that a stripe write involves several RAID
+disks and it's possible the writes don't hit all RAID disks yet before the
+unclean shutdown. We call an array degraded if it has inconsistent data. MD
+tries to resync the array to bring it back to normal state. But before the
+resync completes, any system crash will expose the chance of real data
+corruption in the RAID array. This problem is called 'write hole'.
+
+The write-through cache will cache all data on cache disk first. After the data
+is safe on the cache disk, the data will be flushed onto RAID disks. The
+two-step write will guarantee MD can recover correct data after unclean
+shutdown even the array is degraded. Thus the cache can close the 'write hole'.
+
+In write-through mode, MD reports IO completion to upper layer (usually
+filesystems) after the data is safe on RAID disks, so cache disk failure
+doesn't cause data loss. Of course cache disk failure means the array is
+exposed to 'write hole' again.
+
+In write-through mode, the cache disk isn't required to be big. Several
+hundreds megabytes are enough.
+
+write-back mode
+===============
+
+write-back mode fixes the 'write hole' issue too, since all write data is
+cached on cache disk. But the main goal of 'write-back' cache is to speed up
+write. If a write crosses all RAID disks of a stripe, we call it full-stripe
+write. For non-full-stripe writes, MD must read old data before the new parity
+can be calculated. These synchronous reads hurt write throughput. Some writes
+which are sequential but not dispatched in the same time will suffer from this
+overhead too. Write-back cache will aggregate the data and flush the data to
+RAID disks only after the data becomes a full stripe write. This will
+completely avoid the overhead, so it's very helpful for some workloads. A
+typical workload which does sequential write followed by fsync is an example.
+
+In write-back mode, MD reports IO completion to upper layer (usually
+filesystems) right after the data hits cache disk. The data is flushed to raid
+disks later after specific conditions met. So cache disk failure will cause
+data loss.
+
+In write-back mode, MD also caches data in memory. The memory cache includes
+the same data stored on cache disk, so a power loss doesn't cause data loss.
+The memory cache size has performance impact for the array. It's recommended
+the size is big. A user can configure the size by::
+
+	echo "2048" > /sys/block/md0/md/stripe_cache_size
+
+Too small cache disk will make the write aggregation less efficient in this
+mode depending on the workloads. It's recommended to use a cache disk with at
+least several gigabytes size in write-back mode.
+
+The implementation
+==================
+
+The write-through and write-back cache use the same disk format. The cache disk
+is organized as a simple write log. The log consists of 'meta data' and 'data'
+pairs. The meta data describes the data. It also includes checksum and sequence
+ID for recovery identification. Data can be IO data and parity data. Data is
+checksummed too. The checksum is stored in the meta data ahead of the data. The
+checksum is an optimization because MD can write meta and data freely without
+worry about the order. MD superblock has a field pointed to the valid meta data
+of log head.
+
+The log implementation is pretty straightforward. The difficult part is the
+order in which MD writes data to cache disk and RAID disks. Specifically, in
+write-through mode, MD calculates parity for IO data, writes both IO data and
+parity to the log, writes the data and parity to RAID disks after the data and
+parity is settled down in log and finally the IO is finished. Read just reads
+from raid disks as usual.
+
+In write-back mode, MD writes IO data to the log and reports IO completion. The
+data is also fully cached in memory at that time, which means read must query
+memory cache. If some conditions are met, MD will flush the data to RAID disks.
+MD will calculate parity for the data and write parity into the log. After this
+is finished, MD will write both data and parity into RAID disks, then MD can
+release the memory cache. The flush conditions could be stripe becomes a full
+stripe write, free cache disk space is low or free in-kernel memory cache space
+is low.
+
+After an unclean shutdown, MD does recovery. MD reads all meta data and data
+from the log. The sequence ID and checksum will help us detect corrupted meta
+data and data. If MD finds a stripe with data and valid parities (1 parity for
+raid4/5 and 2 for raid6), MD will write the data and parities to RAID disks. If
+parities are incompleted, they are discarded. If part of data is corrupted,
+they are discarded too. MD then loads valid data and writes them to RAID disks
+in normal way.