When is the M.2 SSD cache beneficial?

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Beau
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Joined: Tue Sep 10, 2019 2:14 am

When is the M.2 SSD cache beneficial?

Post by Beau »

I would like some help to understand if I would benefit from adding M.2 SSDs to my new NAS. I understand the concept of caching as a ME/SW engineer but am not familiar with the hardware standards and their implementations. I have a AS6704T (RAID 5) with 4 Seagate Exos 18x HDDs.

My primary application is storing my photos the RAW files are ~80MB each so ~8GB per 100. There are also small files XMP generated while processing/editing but not a big data transfer issue, so less clear about impact of small file transfers.

It seems to me that if the write cache was initially empty and I could upload 100 photos at SSD speeds (~3000-5000 MBs rather than SATA HDD 600 MBs) that would be a big (~5x-10x) benefit.

This would stop being a benefit if the write cache was full and needed to be flushed to HDDs for more writes. But if the NAS would flush the SSD write cache to HDD when it was idle, it would again be a big benefit. again empty when the next upload happened. Does this happen, or does it remain full and just flush oldest files as needed?

There might be some benefit to a read cache during editing but I don’t have a good sense of the impact since it is not a file size/volume limit. There would likely be a benefit to catalog (directory type) activities like cached thumbnail images. I usually do not need to edit the same photo multiple times so think the read cache is less valuable?

Any information would be helpful.

BTW This information would make a great Asustor College doc for other non-IT users.

Thanks
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orion
Posts: 3482
Joined: Wed May 29, 2013 11:09 am

Re: When is the M.2 SSD cache beneficial?

Post by orion »

You'll gain big performance benefits with write cache when file size is not larger than write cache size. And, yes, it's hard to see benefits when write cache is full and you are still transferring files. Access time of M.2 devices is usually small (of course, bandwidth is usually larger). It means you'll feel much faster when you read data from M.2 devices. So I think read cache hit is the most useful thing.

And I actually prefer to create a RAID-5 volume on those 4 M.2 devices. I can setup a sync job to transfer import files to HDD RAID-5 volume monthly.
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