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July 22nd, 2015 02:00
VNX Fast Cache
Dear all
I am a little bit p*** then our sales sold some VNX with fast cache but only 2 Flash disks. Based on my understanding that is not a good idea at all! We are creating a RAID 1 out of 2 disks which means we have read/write cache in addition. My questions now:
1) Is write and read cache mirrored (whole disk) or only write?
2) Will it try to fill up the cache allways to 100%? Can i see how many data is in there?
3) What happens when 1 disk breaks? Based on the whitepaper i will be degraded and only be usable for read.
4) Further it tells me if there is no hot spare for this Fast Cache SSD RAID Group it will also be in degraded state and only serve read. Is this right?
5) On my understanding SSD are more or less easy to measure regarding outtage, Per disk lets say 100'000 accesses so if the SSD is mirrored it should be more or less the same lifetime for both disks isn't it? How high is the risk that both disks die at the same time?
6) Is there are a flush process from SSD to Vault in case of power outage?
Thank you very much :-)
brettesinclair
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July 22nd, 2015 04:00
1. FAST Cache is R/W and mirrored in RAID 1.
2. FAST Cache should always be highly utilized, especially when you only have a relatively small size of 2 disks. Generally, I find the best metrics are the hit ratios, which you can measure through analyzer, M & R or even Unisphere Central.
3. In a 2 disk RAID 1, 1 drive failure degrades the Cache and it then serves reads only until it's fixed.
4. Correct, but you should have a spare, so the impact would then be minimal.
5. The FAST Cache drives are SLC so endurance is well rated. The MTBF stats are broad stats only, failures of components can just happen at anytime. Although, I have found the drives used to be very reliable.
6. FAST Cache is non-volatile so no flush/destage process is required.
DukeR1337
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July 22nd, 2015 05:00
Thank you. Is the hotspare still required in Gen2 it is not that clear in the white papers.
brettesinclair
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715 Posts
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July 22nd, 2015 05:00
You don't have to use spares, but IMO they should always be used.
DukeR1337
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208 Posts
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July 22nd, 2015 06:00
Ok so we dont have to use spare in gen1 and gen2 but it is always recommended. It was a little bit unclear in the white papers.
Thank you