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

[keguang xu]

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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