Tag Archives: Dedup

SSD Dedup and VDI

I found this nice Symantec blog about the SSD+Dedup+VDI issues in the DCIG site. Basically I agree with its main claim that SSD+Dedup is a good match for VDI. On the other side, I think that the 3 potential “pitfalls” mentioned in the post are probably relevant for a naive storage system, and much less for an enterprise level disk array. Here is why (the blue parts are citations from the original post):

  • Write I/O performance to SSDs is not nearly as good as read I/Os. SSD read I/O performance is measured in microseconds. So while SSD write I/O performance is still faster than writes to hard disk drives (HDDs), writes to SSDs will not deliver nearly the same performance boost as read I/Os plus write I/O performance on SSDs is known to degrade over time.
This claim is true only for non enterprise level SSDs. Enterprise level SSDs write performance suffer much less from performance degradation and due its internal NVRAM, the write latency is as good as read latency, if not better. Furthermore most disk arrays have non trivial logic and enough resources to handle these issues even if the SSDs cannot.
  • SSDs are still 10x the cost of HDDs. Even with the benefits provided by deduplication an organization may still not be able to justify completely replacing HDDs with SSDs which leads to a third problem.
There is no doubt that SSDs are at least 10x more expensive than HDDs in terms of GB/$. But when comparing the complete solution cost the outcome is different. In many VDI systems the real main storage constrain is IOPS and not capacity. This means that a  HDD based solution  may need to over provision the system capacity and/or use small disks such that you will have enough (HDD) spindles to satisfy the IOPS requirements. In this case, the real game is IOP/$ where SSDs win big time. Together with the Dedup oriented space reduction, the total solution’s cost maybe very attractive.
  • Using deduplication can result in fragmentation. As new data is ingested and deduplicated, data is placed further and further apart. While fragmentation may not matter when all data is stored on SSDs, if HDDs are still used as part of the solution, this can result in reads taking longer to complete.

Basically I agree, but again the disk array logic may mitigate at least some of the problem. Of course 100% SSD solution is better (much better is some cases). but the problem is that such solutions are still very rare if at all.

Advertisements

Leave a comment

Filed under Enterprise Storage, ssd, Storage architectures, VDI, Virtualization