Re: [Xen-users] LVM or file storage?

[Boris Senker <senker@xxxxxxxxx>]

> Guys,

thanks for your replies.


> The reason loop appears to not generate as much load is that it is not
> writing your data out to disk. It is cached in the memory by the loop
> driver and only flushed periodically. Needless to say this is playing
> russian roulette with your data - if you experiance an outage on Dom0
> chances are that your guest filesystems will experiance *catastrophic*
> data loss. Not even journalling in the guest FS will help you here
> since the journall writes will simply be cached in memory in the loop
> driver.
I'd definitely opt for reliable data storage, for the guests to be properly
restorable at any point.


> If you want to compare performance of real block devices, vs a file
> backed image use the blocktap driver instead of the loop driver.
>
> eg Instead of
>
>    file:/path/to/image/file,xvda,w
>
> Use the path like
>
>    tap:aio:/path/to/image/file,xvda,w
Will surely try.


> Also, I'd recommend fully allocating the disk space for your file image,
> rather than using sparse files - there is significant overhead involved
> in extending the sparse files at runtime which can lead to unexpected
> performance degradation. Sparse is fine for development/testing, but in
> production you want non-sparse files.
Actually all file images were done with full allocations already. Saw 
the sparse vs. full
discussion somewhere before.

> Not to mention some OSes don't support sparse files, like NetBSD.
>
> For my machines I use LVM, it makes backups easier and giving a VM 
> more space is a simple lvextend/growfs operation.  You can always 
> run LVM on top of RAID, at least that will give you some redundancy.
This is what attracted me towards LVM (especially in regards to that 
150GB Samba partition). I am running LVM on top of raid already - the 
LV group and volumes were initially created on a /dev/md1.
But honesty, I am still not sure what storage type to incline towards.

Thanks again,

Boris


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