Re: [Libguestfs] [PATCH nbdkit 5/6] cow: Implement efficient trimming support.
On 1/26/21 3:51 PM, Richard W.M. Jones wrote:
By storing an extra bit in the overlay bitmap we can mark when whole
blocks have been trimmed. This allows us to match a feature of qcow2
(zero clusters).
In theory we could punch holes in the overlay to save a bit of disk
space.
---
filters/cow/blk.h | 4 ++
filters/cow/blk.c | 106 +++++++++++++++++++++++++++++++---------------
filters/cow/cow.c | 75 +++++++++++++++++++++++++++++++-
3 files changed, 149 insertions(+), 36 deletions(-)
+++ b/filters/cow/cow.c
@@ -150,7 +150,7 @@ cow_can_write (struct nbdkit_next_ops *next_ops, void *nxdata, void
*handle)
+/* Trim data. */
+static int
+cow_trim (struct nbdkit_next_ops *next_ops, void *nxdata,
+ void *handle, uint32_t count, uint64_t offset, uint32_t flags,
+ int *err)
+{
+ CLEANUP_FREE uint8_t *block = NULL;
+ uint64_t blknum, blkoffs;
+ int r;
+
+ if (!IS_ALIGNED (count | offset, BLKSIZE)) {
+ block = malloc (BLKSIZE);
+ if (block == NULL) {
+ *err = errno;
+ nbdkit_error ("malloc: %m");
+ return -1;
+ }
+ }
Technically, we could leave the unallocated portions unchanged rather
than forcing it to read as zero. Maybe it's worth a command-line option
- some users want security (no stale data can leak) while others want
speed (no time wasted on read-modify-write).
Overall, the series looks good (and yes, I know you pushed before my
review, which turned out fine).
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org