Xen users - Oct 2011 - Intellicache linked to vhd corruption

Good day

I''m using XCP 1.1 and have been testing out Intellicache. The VM
I''m
experimenting with is a Windows 7 machine, running from an NFS SR. I have
enabled caching on the XCP host, pointing to a new EXT3 SR on an Intel SSD.
I then enabled caching of what is the VM''s c: with this command:

xe vdi-param-set uuid=75eaac3e-d2ec-4f45-ac9f-67586f2d703c
allow-caching=true on-boot=persist

In testing, performance has been astonishing. Naturally, writes are limited
by NFS, but the raw speed boost we''ve seen is incredible. I''d
dearly love to
leave Intellicache enabled, but I''m seeing predictable, regular
corruption.
I have a VM protection policy enabled, set to perform a simple disk snapshot
each day at 23:45. After snapshotting, the cached vhd is, according to
Windows, corrupt. A read-only chkdsk reports hundreds of errors before
failing to continue in read-only mode.

I have now shut the VM down, disabled caching, and started it up. Windows
automatically scheduled a chkdsk on c: and is repairing as I type this. I
had hoped the "corruption", if that really is the case, would not have
hit
the c:, but it has.

Searching online, I''ve found zero reported cases of this type of
behaviour.
Does anyone here have an idea how and why corruption sneaks in?

Some notes:
I''m using xapi_version_override to emulate XenServer 5.6.100
The VM in question has dynamic memory allocated
VM protection policy is enabled, performing a daily, disk-based snapshot


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Hi, Florian

The documentation with Intellicache info can be found here:
support.citrix.com/servlet/KbServlet/download/28750-102-661359/XenServer-6.0.0-installation.pdf

Most commands and descriptions are there though I''ve heard, somewhere,
that
resizing a cache-enabled VDI will break that VDI. If this is true, it''s
scarily not mentioned in the documentation.

Not snapshotting is simply not an option; it''s one of the reasons we
virtualise. Disabling caching before performing a snapshot is simply not
possible, as adjusting the caching parameter is only possible on a halted
VM.


On 25 October 2011 00:10, Florian Heigl <florian.heigl@gmail.com> wrote:
> Hi,
>
> a not immediately helpful question:
>
> Where did you even find the documentation about "Intellicache".
> Ever since XCP 1.0 I''ve been asking people about that storage
caching
> from the release notes and got mixed reactions, mostly blank stares,
> but all not what you''d call an answer :)
>
> Also, please don''t be offended but the obvious answers are:
> - do not trigger snapshots
> - detach the cache prior to the snapshot, snapshot, attach afterwards.
>
> This might include a lot of UUID juggling, one of the reasons why
I''ll
> never use XenServer or XCP in production. :>
> But I hope you''ll get an answer to this.
>
> Florian
>

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