Applying with these fixes on my side:
- the example: a whole 1M cluster [0, 2048) built from bios [0, 1024)
and [2048, 3072), instead of [0, 512) and [1024, 1536)
- "range in the file" -> "range of the device", "always" dropped
- added why the limit is USHRT_MAX and what the user sees, split into
paragraphs
On Mon, 09 Nov 2026 18:36:19 +0300, Andrey Zhadchenko wrote:
> Imagine we have a request with starting sector 0 and length 1024,
> so read or write means [0, 1024) range in the file. For discard this
> request can have 2 bios [0, 512) and [1024, 1536), but we always
> discarded [0, 1024).
ploop_handle_discard_pio() ignores a discard that is not a whole
cluster, so with the default 1M cluster this request discards nothing.
Data is lost only when a gapped request adds up to a whole cluster. The
message now reads:
Discard is a special type of request that can have non-contiguous
bios: dm-ploop keeps the dm stacking default max_discard_segments =
USHRT_MAX, so the block layer merges discard bios with gaps.
Imagine we have 1M clusters and a request with starting sector 0 and
length 2048 sectors, so read or write means the [0, 2048) range of
the device. For discard this request can have 2 bios [0, 1024) and
[2048, 3072), but we discarded the whole cluster [0, 2048).
Data in sectors 1024-2047 reads back as zeroes.
Set max_discard_segments to 1 to avoid such merges.
--
Konstantin Khorenko,
Virtuozzo Linux Kernel Team
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