Hello T Masteen,
I scanned through my personal notes and was happy enough to find this info I once got from SpecV development wrt this very topic:
Usually lower cache does not have to mirror the data to the partner
node as the data has already been mirrored in upper cache.
However, compression is done between upper and lower cache,
and it modifies the data.
This means that lower cache has to mirror the data across the inter-site link again.
As others had correctly mentioned before, compression indeed happens in between Upper Cache and Lower Cache.
I had assumed hence, Upper & Lower Cache would need to be copied between the nodes in an Enhanced Stretched Cluster with DRP compression.
To be 100% sure on that, I have double-checked with development team on this, and they confirmed this to be the case.
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Christian Schroeder
IBM SpecV Storage Support with Passion
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Original Message:
Sent: Fri May 21, 2021 01:44 PM
From: T Masteen
Subject: SVC compression in DRP and Enhanched Stretched Cluster
Hello Christian, Thanks for looking into it.
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T Masteen
Original Message:
Sent: Fri May 21, 2021 08:43 AM
From: Christian Schroeder
Subject: SVC compression in DRP and Enhanched Stretched Cluster
T Masteen's observation might be correct.
For the now legacy RTC (Real-Time Compression) in fact the benefit of one-time traversal of write data between the nodes in an Enhanced Stretched Cluster does not apply. I strongly assume this equally applies to DRP compression as well, need to verify this.
For RTC this used to be documented somewhere (IBM Docs alias Knowledge Center, IBM Redbooks...), yet I didn't get hold of this for now, will post an update once I got it.
Best regards,
Christian Schroeder
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Christian Schroeder
IBM SpecV Storage Support with Passion
Original Message:
Sent: Thu May 20, 2021 03:04 AM
From: Tom Van Daele
Subject: SVC compression in DRP and Enhanched Stretched Cluster
I don't really see why you would have more or less bandwidth usage on your ISL's by using compressing in SVC ESC since you are basically constantly mirroring the write cache between the split IO group.
Compression is one the last steps performed before actually commiting the write to disk, so the bandwith from SVC to your back end system could be smaller since there the data is compressed. I think mirroring your write cache happens before that.
I know there is a schema on how Spectrum Virtualize handles an IO and what steps are taken when but I can't seem to find it.
Br,
Tom
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Tom Van Daele
Original Message:
Sent: Thu April 22, 2021 02:44 PM
From: T Masteen
Subject: SVC compression in DRP and Enhanched Stretched Cluster
Hello all,
With SVC Enhanched Stretched Cluster when writing there is reduced inter-site traffic (compared with a classic stretched cluster).
However when using compression lower cache will be re-mirrored. Hence additional bandwidth.
Now we have compression in Data Reduction Pools. Is there a difference in how writes are handled in a ESC?
Thanks!
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T Masteen
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