Re: [PATCH v2 pve-qemu 1/5] add optional no-cache flag to bypass host page cache on pbs-restore

[Fiona Ebner <f.ebner@proxmox.com>]

I'd move the pbs-restore as a prefix to the front in the title, i.e.
pbs-restore: add optional ...

Am 19.08.26 um 4:47 PM schrieb Christian Ebner:
> Signed-off-by: Christian Ebner <c.ebner@proxmox.com>

Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>

with one more comment below

> ---
> changes:
> - no changes
> 
>  ...no-cache-flag-to-skip-host-page-cach.patch | 64 +++++++++++++++++++
>  debian/patches/series                         |  1 +
>  2 files changed, 65 insertions(+)
>  create mode 100644 debian/patches/pve/0047-pbs-restore-add-no-cache-flag-to-skip-host-page-cach.patch
> 
> diff --git a/debian/patches/pve/0047-pbs-restore-add-no-cache-flag-to-skip-host-page-cach.patch b/debian/patches/pve/0047-pbs-restore-add-no-cache-flag-to-skip-host-page-cach.patch
> new file mode 100644
> index 0000000..435aab3
> --- /dev/null
> +++ b/debian/patches/pve/0047-pbs-restore-add-no-cache-flag-to-skip-host-page-cach.patch
> @@ -0,0 +1,64 @@
> +From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
> +From: Christian Ebner <c.ebner@proxmox.com>
> +Date: Mon, 17 Aug 2026 10:21:58 +0200
> +Subject: [PATCH] pbs-restore: add no-cache flag to skip host page cache on
> + restore
> +
> +Optional flag which allows to set BDRV_O_NOCACHE on the block
> +backend, forcing to skipping the host page cache while restoring.
> +
> +Allows to explicitly skip the page cache for cases where it can cause
> +issues like restoring to ZVOLs where page writeback can cause I/O
> +delay issues in other ZFS-backed VMs on certain setups.

Here you could go a bit more into detail and maybe also provide a
reference to the issues

> +
> +Keeping it opt-in for full backwards compatibility, allowing to only
> +enable it on targets where required.
> +
> +Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
> +---
> + pbs-restore.c | 11 +++++++++++
> + 1 file changed, 11 insertions(+)
> +
> +diff --git a/pbs-restore.c b/pbs-restore.c
> +index f165f418af..55a3cb235d 100644
> +--- a/pbs-restore.c
> ++++ b/pbs-restore.c
> +@@ -81,6 +81,7 @@ int main(int argc, char **argv)
> +     const char *keyfile = NULL;
> +     int verbose = false;
> +     bool skip_zero = false;
> ++    bool no_cache = false;
> + 
> +     error_init(argv[0]);
> + 
> +@@ -93,6 +94,7 @@ int main(int argc, char **argv)
> +             {"repository", required_argument, 0, 'r'},
> +             {"ns", required_argument, 0, 'n'},
> +             {"keyfile", required_argument, 0, 'k'},
> ++            {"no-cache", no_argument, 0, 'N'},
> +             {0, 0, 0, 0}
> +         };
> +         int c = getopt_long(argc, argv, "hvf:r:k:", long_options, NULL);
> +@@ -124,6 +126,9 @@ int main(int argc, char **argv)
> +             case 'S':
> +                 skip_zero = true;
> +                 break;
> ++            case 'N':
> ++                no_cache = true;
> ++                break;
> +             case 'h':
> +                 help();
> +                 return 0;
> +@@ -198,6 +203,12 @@ int main(int argc, char **argv)
> +     }
> +     Error *local_err = NULL;
> +     int flags = BDRV_O_RDWR;
> ++    if (no_cache) {
> ++        flags |= BDRV_O_NOCACHE;
> ++        if (verbose) {
> ++            fprintf(stderr, "skip host page cache for restore of '%s'\n", target);
> ++        }
> ++    }
> +     BlockBackend *blk = blk_new_open(target, NULL, options, flags, &local_err);
> +     if (!blk) {
> +         fprintf(stderr, "%s\n", error_get_pretty(local_err));
> diff --git a/debian/patches/series b/debian/patches/series
> index 4f9ed75..6ef37f4 100644
> --- a/debian/patches/series
> +++ b/debian/patches/series
> @@ -76,3 +76,4 @@ pve/0043-PVE-backup-get-device-info-allow-caller-to-specify-f.patch
>  pve/0044-PVE-backup-implement-backup-access-setup-and-teardow.patch
>  pve/0045-PVE-backup-prepare-for-the-switch-to-using-blockdev-.patch
>  pve/0046-savevm-async-reuse-migration-blocker-check-for-snaps.patch
> +pve/0047-pbs-restore-add-no-cache-flag-to-skip-host-page-cach.patch