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

Ritesh Harjani
2024-03-06T13:13:47Z

John Garry <john.g.garry@oracle.com> writes:

> 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

Thanks John for giving this a try!

> suffers from the same issue discussed at
> https://lore.kernel.org/linux-fsdevel/434c570e-39b2-4f1c-9b49-ac5241d310ca@oracle.com/
> with regards to writing to partially written extents, which I have tried
> to address properly in my v2 for that same series.

I did go through other revisions, but I guess I missed going through this series.

Thanks Dave & John for your comments over the series.
Let me go through the revisions I have missed and John's latest revision.
I will update this series accordingly.

Appreciate your help!
-ritesh