Re: [RFC 0/9] ext4: Add direct-io atomic write support using fsawu

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On 02/03/2024 07:41, Ritesh Harjani (IBM) wrote:
Hello all,

This RFC series adds support for atomic writes to ext4 direct-io using
filesystem atomic write unit. It's built on top of John's "block atomic
write v5" series which adds RWF_ATOMIC flag interface to pwritev2() and enables
atomic write support in underlying device driver and block layer.

This series uses the same RWF_ATOMIC interface for adding atomic write support
to ext4's direct-io path. One can utilize it by 2 of the methods explained below.
((1)mkfs.ext4 -b <BS>, (2) with bigalloc).

Filesystem atomic write unit (fsawu):
============================================
Atomic writes within ext4 can be supported using below 3 methods -
1. On a large pagesize system (e.g. Power with 64k pagesize or aarch64 with 64k pagesize),
    we can mkfs using different blocksizes. e.g. mkfs.ext4 -b <4k/8k/16k/32k/64k).
    Now if the underlying HW device supports atomic writes, than a corresponding
    blocksize can be chosen as a filesystem atomic write unit (fsawu) which
    should be within the underlying hw defined [awu_min, awu_max] range.
    For such filesystem, fsawu_[min|max] both are equal to blocksize (e.g. 16k)

    On a smaller pagesize system this can be utilized when support for LBS is
    complete (on ext4).

2. EXT4 already supports a feature called bigalloc. In that ext4 can handle
    allocation in cluster size units. So for e.g. we can create a filesystem with
    4k blocksize but with 64k clustersize. Such a configuration can also be used
    to support atomic writes if the underlying hw device supports it.
    In such case the fsawu_min will most likely be the filesystem blocksize and
    fsawu_max will mostly likely be the cluster size.

    So a user can do an atomic write of any size between [fsawu_min, fsawu_max]
    range as long as it satisfies other constraints being laid out by HW device
    (or by software stack) to support atomic writes.
    e.g. len should be a power of 2, pos % len should be naturally
    aligned and [start | end] (phys offsets) should not straddle over
    an atomic write boundary.

JFYI, I gave this a quick try, and it seems to work ok. Naturally it suffers from the same issue discussed at https://lore.kernel.org/linux-fsdevel/434c570e-39b2-4f1c-9b49-ac5241d310ca@xxxxxxxxxx/ with regards to writing to partially written extents, which I have tried to address properly in my v2 for that same series.

Thanks,
John




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