RE: [Xen-users] Xen Sparse Image Files and MaxIQ SSD

[Andy Pace <APace@xxxxxxxxxxxxx>]

I think we're getting a bit off topic. We have not noticed any significant 
performance *DECREASE* using sparse image files. We're getting gigabit line 
speed writes (130MB/s).

I welcome anyone elses comments or feedback in regards to the MaxIQ SSD 
question.

-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of George Shuklin
Sent: Thursday, July 08, 2010 6:37 PM
To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-users] Xen Sparse Image Files and MaxIQ SSD

What data exist in zero-length file? In sparsed file 'holes' areas does not 
exists on lower levels, this just 'imagination' of filesystem. The areas with 
data are threating normally and would cached to MaxIQ only if reed often.

In other words, MaxIQ works on block level. And any block level have no idea 
about sparsed files or filesystem or any other swaps created on block device. 
Read requests to empty area will not come to block level.
Read requests to filled area and write requests to any area will come as they 
come for any other files.

Those two things just have no any connection. (like your ethernet card does not 
care about ssh key length in any way).

But sparsed files in product have a great performance penalty - in my tests any 
write operations to sparsed areas reduce performance about twice. Most visible 
test is filesystem creation. And in long terms many sparsed files on same 
storage means higher fragmentation (and more random io operations, and 
performance loss).

So, disk space is cheap, filesystems are resizable, so no reason for using 
sparse in production environment.

N.B. For SSD this can be other question, I'm talking about old pain HDD (and 
even MaxIQ does not change it principally, it just an emulation of big memory 
cache).

Ð ÐÑÐ, 08/07/2010 Ð 16:56 -0500, Andy Pace ÐÐÑÐÑ:
> Right, but the data does exist. What I'm wondering is if it will treat the 
> sparse image file as 1 file, or will it cache on a block basis.
> 
> > Do you often read holes from sparse?
> 
> I'm not sure how to answer that.
> 
> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of George 
> Shuklin
> Sent: Thursday, July 08, 2010 4:36 PM
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Xen Sparse Image Files and MaxIQ SSD
> 
> That's simple: caching only if data  exists.
> 
> All 'holes' in spare file does not exists. They are just opinion of 
> filesystem, but on block level there is no sparse.
> 
> One more: MaxIQ cache most reed data. Do you often read holes from sparse?
> 
> 
> Ð ÐÑÐ, 08/07/2010 Ð 16:08 -0500, Andy Pace ÐÐÑÐÑ:
> > Hello list!
> > 
> >  
> > 
> > Iâm currently storing Xen sparse image files for VMâs on remote 
> > attached storage devices, and looking to increase the amount of IO 
> > available on it. Iâve ran across MaxIQ SSD read acceleration 
> > caching, and was wondering if this is a smart move, given the way we 
> > use the sparse image files.
> > 
> >  
> > 
> > My fear is that the MaxIQ will cache entire image files, instead of 
> > the most commonly accessed files *INSIDE* the VM. Can anyone confirm 
> > or deny my theory?
> > 
> >  
> > 
> > Iâve talked with Adaptec and this is what they told me:
> > 
> >  
> > 
> > --------
> > 
> > It's hard to say exactly. Keep in mind that a RAID card isn't aware 
> > of files, only blocks. So depending on how the blocks are accessed, 
> > the MaxIQ product may or may not help you. My first impression with 
> > 30GB files is that it may not be very helpful.
> > 
> > --------
> > 
> >  
> > 
> > So, that being said, right now we are accessing the image files 
> > using the âfileâ mode:
> > 
> >  
> > 
> >     <disk type='file' device='disk'>
> > 
> >       <driver name='file'/>
> > 
> >       <source file='/distributed/vms/29609/disk.img'/>
> > 
> >       <target dev='xvda' bus='xen'/>
> > 
> >     </disk>
> > 
> >  
> > 
> > Will the MaxIQ work in the way I want it to, or will I have to 
> > change the way we access the images? Or will it end up trying to 
> > cache entire images onto the SSD? Has anyone had experience with 
> > this type of application?
> > 
> > 
> > _______________________________________________
> > Xen-users mailing list
> > Xen-users@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-users
> 
> 
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