Re: [Xen-users] Just some (probably useless) information

[Steven Wilcoxon <stevew@xxxxxxxxx>]

FYI, by default dd work with very small blocks of data. For higher performance you can try:

dd bs=64k <in >out

or

dd bs=64k <in | dd bs=64k >out

I've even seen some systems having an additional dd in the middle will speed things up by buffering data to keep the write going constantly. Which is the fastest varies depending on the h/w configuration and if multiple drives are being used.

S.W.

On 3/21/2010 6:03 AM, Grant McWilliams wrote:

On Sat, Mar 20, 2010 at 12:37 AM, Brandon Lamb <brandonlamb@xxxxxxxxx> wrote:

I did a very non scientific test for speeds of backing up disk/file
images to disk/file backups

I used lvm for both the disk based and to put the files onto during
the test. I used four 640gig sata 3.0gbps drives in a raid 10. There
was 2083 megabytes used on the filesystem, a windows xp install.

dd disk -> disk = 36 minutes

ntfsclone disk -> disk = 4 minutes

ntfsclone disk -> file = 2 minutes

cp file -> file = 11 seconds

So now I am left with the question of do I use file based images which
will have slower performance but super fast backup times, or go with a
lvm -> disk/file for the backup.

My particular use case will be a user gets a 20 gig drive for xp,
their it guy will do manual image backup creation. If they blow up
their windows box we will have a "click click" ok now they are running
againback up using the backup disk/file image. If I use all disk based
images then I will have to also create a file backup of the backup
disk for my remote backup box to save in case the host dies.

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How big was the actual LV because it definitely shouldn't take dd 40 minute to copy 2 GB! Without optimizations that size disk should take 1 minute. If you go with ddrescue or tune dd with large blocks etc.. and depending on the speed of your RAID 10 you should be able to get that down to 30 seconds or less. If your volume is a lot larger than 2GB then you need to play with the sparse options but I think that only works with output files. Still when dding from one 80 GB drive to another 80GB drive it takes me 40 minutes or roughly 2/3 the maximum speed of the drive.

The other option would be to dd the LV to a backup LV and then have it mounted on another DomU (a backup VM) and just rsync to it often. This would keep an up to date LV which you could detatch and use in a hurry on the other VM. Of course you'd still want to follow the normal backup process anyway so you have more than one level of backup.


Grant McWilliams

Some people, when confronted with a problem, think "I know, I'll use Windows."
Now they have two problems.

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