Re: [Libguestfs] [nbdkit PATCH] file: Add trim support

We already have support in the file driver for punching holes during .zero with the may_trim flag (via FALLOC_FL_PUNCH_HOLE), so we should use the same mechanism to support .trim. Note that the NBD spec says that trim is advisory (we can return success even if we did nothing); but at the same time, it's nicer to avoid advertising the feature if we know for sure we can't do it, so we also need .can_trim. Note that there's still an element of runtime behavior, as FALLOC_FL_PUNCH_HOLE doesn't work on all filesystems; there we fall back on the NBD protocol allowing us to be advisory for all but a handful of errno values that we can directly report back over NBD. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- I couldn't test --filter=log results on trim commands without at least one plugin that supports trim ;) plugins/file/file.c | 47 ++++++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 44 insertions(+), 3 deletions(-) diff --git a/plugins/file/file.c b/plugins/file/file.c index 1fe4191..081848b 100644 --- a/plugins/file/file.c +++ b/plugins/file/file.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2013 Red Hat Inc. + * Copyright (C) 2013-2018 Red Hat Inc. * All rights reserved. * * Redistribution and use in source and binary forms, with or without @@ -175,6 +175,18 @@ file_get_size (void *handle) return statbuf.st_size; } +static int +file_can_trim (void *handle) +{ + /* Trim is advisory, but we prefer to advertise it only when we can + * actually (attempt to) punch holes. */ +#ifdef FALLOC_FL_PUNCH_HOLE + return 1; +#else + return 0; +#endif +} + /* Read data from the file. */ static int file_pread (void *handle, void *buf, uint32_t count, uint64_t offset) @@ -219,7 +231,7 @@ file_pwrite (void *handle, const void *buf, uint32_t count, uint64_t offset) return 0; } -/* Write data to the file. */ +/* Write zeroes to the file. */ static int file_zero (void *handle, uint32_t count, uint64_t offset, int may_trim) { @@ -268,6 +280,33 @@ file_flush (void *handle) return 0; } +/* Punch a hole in the file. */ +static int +file_trim (void *handle, uint32_t count, uint64_t offset) +{ + int r = -1; +#ifdef FALLOC_FL_PUNCH_HOLE + struct handle *h = handle; + + /* Trim is advisory; we don't care if it fails for anything other + * than EIO or EPERM. */ + r = fallocate (h->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, + offset, count); + if (r < 0) { + if (errno != EPERM && errno != EIO) { + nbdkit_debug ("ignoring failed fallocate during trim: %m"); + r = 0; + } + else + nbdkit_error ("fallocate: %m"); + } +#else + /* Based on .can_trim, this should not be reached. */ + errno = EOPNOTSUPP; +#endif + return r; +} + static struct nbdkit_plugin plugin = { .name = "file", .longname = "nbdkit file plugin", @@ -279,10 +318,12 @@ static struct nbdkit_plugin plugin = { .open = file_open, .close = file_close, .get_size = file_get_size, + .can_trim = file_can_trim, .pread = file_pread, .pwrite = file_pwrite, - .zero = file_zero, .flush = file_flush, + .trim = file_trim, + .zero = file_zero, .errno_is_preserved = 1, };