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March 09, 2010

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nate

Pretty impressive that 128MB block size.. I had gotten various tidbits of information about what AO would eventually turn out to be for quite a while now and I didn't expect it to be able to scale to a sub chunklet (256MB) level. I didn't even expect it to scale down to the chunklet level originally(on the current T/F systems anyways)

It's hard to imagine the amount of performance metrics to keep track if if your on say a 100TB+ system and things are being monitored at the 128MB level. That's gotta be what at least 2-6 data(read+write) points per 128MB region, and 8 regions per GB. That's just insanity :)

Now what I want to see is some SPC-1 numbers with AO enabled!

Robert Weilheim

Are chunklets still 256MB? I thought the contents of one chunklet resided on a single physical disk. If AO operates on 128MB blocks of data, how can one chunklet have half on a SSD and half on a SATA disk?

I'm also interested to see how AO interoperates with the zero page reclaim, which operates at the 16K level. 256MB, 128MB, 16K...so many different sizes of data 'blocks' flying around the array makes my head spin.

marc farley

Hi Robert, There are a number of differently sized physical and virtual storage elements in an InSerev array and it sounds like you have some familiarity with system internals. Chunklets are still 256MB. They are RAIDed together to create "RAIDlets" or what I like to call Micro-RAID sets and regions are striped across those micro-RAID sets.

Regions don't span tiers as your question implies, the region is copied from the Micro-RAID set on the source (say SATA) and written to another Micro-RAID set on the target (assuming SSDs in this case). At the time the region is copied across tiers, both copies of it are identical, but the copy residing on SSD becomes the "live" copy and the copy on SATA becomes a back-level version. When the region is later demoted off of SSD, it is copied back to it's original tier.

Chunklets are not copied, they are physically associated with the devices they reside on. Regions on the other hand are logical entities that can be relocated by the system. A region contains a subset of the data in a Micro-RAID set, so when a single region is copied from a Micro-RAID set, the other regions occupying it still remain in place and active. Vacant regions are reclaimed by the system.

I know what you are talking about when you say zero page reclaim, but the term doesn't apply exactly because we don't have pages in an InServ array - chunklets, regions and other smaller granules, but no pages to reclaim. We do have both file system-linked reclamation (with Veritas Storage Foundation today) as well as FS-independent reclamation called Thin Persistence. The processes and results differ somewhat.

Getting back to your question about AO and reclamation interoperability: There is none that I'm aware of and I wouldn't expect to see it. AO is used to manage storage for active data and reclamation is used to manage storage that is no longer being used. Region-space that is vacated by AO will be re-used by the system, but I don't think Thin Reclamation or Thin Persistence are part of that process.

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