On 12/11/2015 10:56 AM, Ján Tomko wrote:

On Wed, Dec 02, 2015 at 03:22:38PM -0500, John Ferlan wrote:

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https://bugzilla.redhat.com/show_bug.cgi?id=1270709

When a volume wipe is successful, a volume refresh should be done afterwards to update any volume data that may be used in future volume commands, such as volume resize. For a raw file volume, a wipe would truncate the file and a followup volume resize the capacity may fail because the volume target allocation isn't updated to reflect the wipe activity.

I would expect that after wiping a 200 MB volume with zeros, it would contain 200 MB of zeros and it would not be shrinkable to 50 MB, not even after a volume refresh.

While it seems ftruncate to 0 bytes and back satisfies the documentation of the virStorageVolWipe API: Ensure data previously on a volume is not accessible to future reads the ALG_ZERO description says: VIR_STORAGE_VOL_WIPE_ALG_ZERO = 0 1-pass, all zeroes

Do we need to update it to reflect that there might not be any pass over the old data (which might not happen for non-sparse files either, if the filesystem does not overwrite the same sectors)?

If updating the docs is necessary - that's fine. Would it be better to update (all or some of): 1. virStorageVolWipe 2. virStorageVolWipePattern 3. virStorageVolWipeAlgorithm 4. virsh.pod (vol-wipe) or do you think it's a bug that some backend uses ftruncate() rather than writing zero's to the volume? See commit id '73adc0e5'. I viewed things as - at some point in time it was deemed acceptable to use ftruncate(), so at least make sure "update" the volume data after wipe is done. I think perhaps calling resize is overkill, but also didn't want to miss out on some "other" backend that did something similar - so taking the broad brush rather than the targeted approach. If updating docs is fine, then how about the following wording: "For some storage backends, the use of the pass all zeros algorithm VIR_STORAGE_VOL_WIPE_ALG_ZERO (default for virStorageVolWipe()), will result in a volume truncation rather than writing all zeros to the volume. After the volume is wiped, libvirt will refresh volume data usage statistics, including allocation and capacity." NB: Calling refreshVol after has the same result as if someone does a "virsh vol-info" or "virsh vol-list default --details"...

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Signed-off-by: John Ferlan <jferlan@redhat.com> --- src/storage/storage_driver.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/storage/storage_driver.c b/src/storage/storage_driver.c index bbf21f6..2e59e39 100644 --- a/src/storage/storage_driver.c +++ b/src/storage/storage_driver.c @@ -2436,7 +2436,19 @@ storageVolWipePattern(virStorageVolPtr obj, goto cleanup; }

- ret = backend->wipeVol(obj->conn, pool, vol, algorithm, flags); + if ((ret = backend->wipeVol(obj->conn, pool, vol, algorithm, flags)) < 0) + goto cleanup;

More readable as: if (func() < 0) goto cleanup;

ret = 0; If the return value does not need to be propagated. (Which it should not here, but it is possible in some cases. I will send a patch)

Reviewer dependent I think... Seems some want to see the one line like I have above, while others want to see the multi-line. I used to be in favor of multi-line, mostly because of the need to be careful about where you put that right ")"! In any case, I see more code come through with the single line, so I've retrained my fingers to use that model. Doesn't really matter to me either way and I don't recall seeing anything in the contributor guidelines...

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+ + /* Best effort to refresh the volume data. If unsuccessful, we've already + * wiped the data so there's no going back on that. Best we can do is + * provide some details over what happened and move on + */

The wipe did happen, but if refreshVol fails, there is something seriously wrong with the volume. I think returning -1 and reporting an error is reasonable here.

Ironically I went with WARNING, but would prefer error; however, flipped a virtual coin and went with WARN. I also had, but ditched a patch that would show the sizes on error for volume shrink. That is in storageVolResize rather than: virReportError(VIR_ERR_INVALID_ARG, "%s", _("can't shrink capacity below " "existing allocation")); I had done something like _("cannot shrink capacity of %lld below " "existing allocation of %lld"), abs_capacity, vol->target.allocation But of course the value internally is in bytes, so it looks odd and isn't what the user provided. So I changed to using %lldK and dividing by 1024, but then thought - damn someone that provided M will not be happy... Couldn't win, so I just removed it. John

Jan

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+ if (backend->refreshVol && + backend->refreshVol(obj->conn, pool, vol) < 0) { + VIR_WARN("failed to refresh volume '%s' info after volume wipe", + vol->name); + virResetLastError(); + }