[Users] Snapshot merging and the effect on underlying LV metadata

[Davis, Richard]

A very neat solution.    I like that.
Thank you for sharing.



-----Original Message-----
From: R P Herrold [mailto:herrold at owlriver.com] 
Sent: 27 February 2014 19:20
To: Davis, Richard
Cc: 'users at ovirt.org'
Subject: Snapshot merging and the effect on underlying LV metadata

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On Thu, 27 Feb 2014, Davis, Richard wrote:

> I am being told that unless the "Wipe After Delete" option is set on a 
> vDisk, any subsequent snapshot merging of the related VM will not 
> delete LV metadata (or any data!) from the volume created by the 
> snapshot. Is this correct ? I'm kinda hoping not !

It is my belief a depetion cannot be relied upon to have happened in all cases.  Some options flag sets in lvm ** do ** persist old data, and so our security practice at PMman to treat data on removed LV's as though it persists

There are published reports that instances on other public cloud providers have been deployed with 'non-wiped' drives in the 'slack space'.  Why run the reputational risk?

When we reclaim a LV, we perform a 'renaming' that permits to spot 'dirty' and 'scratched' instances needing wiping.  [we also fill a new VG / PV with LV's indicating it needs wiping, as we do not wish to expose content if a drive is pulled and then re-used after testing when SMART errors appeared, but do not stand up to disqualify a drive]

Later a cron driven process, sensitive to IO load runs.  It builds a list of candidates over a day old, using 'find' and the LV name series showing it is dirty and scratched.  Then in turn by LV found, it fires off a sub-task (when load is low), which in turn performs a 'niced' 'shred' operation on that LV, followed by the 'shred 'zeroing' operation.  When load is too high, it sleeps for a couple of minutes, and re-tries

fragment:
	 $_shredCmd = "ionice -c 3 shred -n \
		".$_num_passes." -z ".$_working_lvm;

Only when that sub-process has completed do we 'rename' and later 'remove' a given LV, to let its space re-enter the assignment pool

- -- Russ herrold

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