[Beowulf] zram usage
Ellis H. Wilson III
ellis at cse.psu.edu
Thu May 1 14:07:19 PDT 2014
On 05/01/2014 03:57 PM, Prentice Bisbal wrote:
> On 05/01/2014 03:43 PM, Joe Landman wrote:
>> On 05/01/2014 03:33 PM, atchley tds.net wrote:
>>> Yes, but I don't want to buy a Dell just to get a NVMe unit. ;-)
>> Not that I am trying to get the dude a Dell ... but think of it as
>> very expensive shrink wrap packaging ... :^
> On the bright side, a Dell box will be easier to open than a plastic
> clam-shell package.
Dell is the only one shipping the Samsung XS1715 to the best of my
knowledge. Please correct me if there is some other place which is.
Samsung's the only one on the market with anything providing NVMe right
now. Should change within the year though.
But, as a word of caution, let's all beat this into our heads now: Flash
is not RAM. Flash is not RAM. Flash is not RAM. Not even slow RAM.
(Ok, NAND Flash is not slow RAM -- maybe you could get away with saying
NOR flash is slow RAM). If it ain't byte-addressable, it ain't RAM.
That's not to say some very specialized (largely sequential, or aligned)
things you do with RAM for the speed could also be done (almost as well)
with Flash (particularly NVMe, and which would provide you higher
capacities). But do not, under any circumstances, expect to mmap flash
or use flash as swap or do something equally terrible and expect it to
perform reasonably. Normal ram accesses do lots of fun un-aligned
things because they can, and flash, NVMe or not, will kick and scream at
Getting back to zram, that's really interesting, and I had only heard of
it in passing and not read up on it until now. It looks like they added
LZO/LZ4 support this past January, so you may have a wide enough variety
of compressors now to find one that works for your use-case.
Then again, it looks like zram is just a staging mechanism (like a
compressed ramdisk or the like) that creates a block device you can use
to push or pull data from. Could be way off on that. So you'd have to
think hard about how you want to use it. Wouldn't be as simple as
"allocate a bunch of memory and the kernel automagically compresses
some/all in the background."
Department of Computer Science and Engineering
The Pennsylvania State University
More information about the Beowulf