[lustre-devel] HELP: difference between fallocate and truncate implementation

Hi Kg.Xu,

> - from LU-13765 <https://review.whamcloud.com/c/fs/lustre-release/+/39342>,
> it explains why move fallocate() from ldiskfs upto OSD layer (due to
> unpredictable credits ahead), however truncate() resides in ldiskfs layer

When fallocate was first introduced (LU-3606) it did called into
ldiskfs_fallocate(). During LU-13765, it was moved to the OSD layer
with the difference being credits extend (osd_extend_restart_trans()).
These changes could have been done under ldiskfs but  it is much more
straightforward and uncomplicated to implement it under OSD. Though it
is not a layering hack.

Thanks,
Arshad

On Tue, Nov 19, 2024 at 2:38 AM <[email protected]> wrote:
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>    1. HELP: difference between fallocate and truncate
>       implementation (keguang xu)
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Mon, 18 Nov 2024 16:11:38 +0800
> From: keguang xu <[email protected]>
> To: [email protected]
> Subject: [lustre-devel] HELP: difference between fallocate and
>         truncate implementation
> Message-ID:
>         <CAF83deS+MC_ZKr9vXix_Js63ZyJ2KkXsSwLg5FeBs-+NziYf3Q@mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Hi Lustre devs,
>
> I came across the code and noticed an implementation difference between
> fallocate and truncate/punch operations under the lustre/osd-ldiskfs/.
> While *osd_fallocate*() is handled in the Lustre OSD layer, *osd_punch*()
> remains in the ldiskfs layer, both of which may relate to multiple
> transactions as their comments suggested.
> - from LU-13765 <https://review.whamcloud.com/c/fs/lustre-release/+/39342>,
> it explains why move fallocate() from ldiskfs upto OSD layer (due to
> unpredictable credits ahead), however truncate() resides in ldiskfs layer
> instead of moving upto OSD layer?
> - LU-10048 <https://review.whamcloud.com/c/fs/lustre-release/+/27488>
> claims to "avoids restarting truncate transaction handles in main
> transaction", while in osd_fallocate_preallocate() it seems there's no such
> concern? Besides, why should we avoid restarting?
>
> Correct me if I misunderstood the code above...Thank you for your input.
>
> Best Regards,
> kg.xu
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