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peppingc

[SOLVED]Cache Drive Recommendations - SATA SSD or 2.5" HDD?

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I'm in the process of building up a unRAID NAS box running on a HP Microserver Gen8 platform.  I only have 4 3.5" HDD bays all taken up with data drives (2 x 4tb & 2 x 5tb).  I can sneak in a cache drive if use a SATA SSD or 2.5" HDD, there is no room for any more 3.5" drives.  I will probably only have a single cache drive and am wondering if a 1TB 5400rpm HDD will be enough or should I spend a bit more and go the SSD route?  There is only a single SATA port free on the mobo and I will have to piggy back power off something else.

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1 hour ago, peppingc said:

Is that recommendation based on write performance considerations?

It will be.

 

you do not actually say what you intend to use the cache drive for?   That might affect your decision on the best way forward.    Also you do not mention if you intend to run with or without parity.

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1 hour ago, itimpi said:

you do not actually say what you intend to use the cache drive for?   That might affect your decision on the best way forward.    Also you do not mention if you intend to run with or without parity.

I'm assuming it will be used for buffering during large file transfers.  I have a lot of data to migrate to this new server, the data is currently on various HDDs spread across multiple machines.  Yes, will run with parity on a single drive.

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1 minute ago, peppingc said:

I'm assuming it will be used for buffering during large file transfers.  I have a lot of data to migrate to this new server, the data is currently on various HDDs spread across multiple machines.  Yes, will run with parity on a single drive.

Is is normally recommended that you do NOT use the cache drive during initial data load as it typically just slows things down unless all the files you intend to copy in a day will fit onto the cache drive (so that mover can move them (comparatively slowly) to the main array overnight) .  Adding files after the initial load can better use the cache drive as then files are only moved to the array during idle periods (typically overnight).   For this purpose a HDD may be sufficient.  However if you intend to run docker containers or VMs then they gain significant performance benefit to having them hosted an an SSD rather than the main array.

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19 hours ago, itimpi said:

IHowever if you intend to run docker containers or VMs then they gain significant performance benefit to having them hosted an an SSD rather than the main array.

Thanks for the explanation.  I did not know it was possible to run VMs from a cache drive.  At the moment this box is purely for file serving purposes but that may change as I learn more about what unRAID can do.  How do you determine the most appropriate cache size for any given array?  I have 2 x 4tb and 2 x 5tb drives in mine with 1 of the 5tb drives being a parity drive so I effectively have 13tb of usable space in my array.

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There is no magic answer to give for the size of a cache drive (if you even bother with one at all) as it dependent on an individual’s usage pattern.    If you are going to use it in its role of buffering writes to the main array then it should have at least enough space to handle a typical days worth of new files on the basis you will let the mover application run overnight to move file to the main array.  Any requirements for applications run as either docker containers and/or VMs are over and above that.

 

 

 

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2 hours ago, itimpi said:

If you are going to use it in its role of buffering writes to the main array then it should have at least enough space to handle a typical days worth of new files on the basis you will let the mover application run overnight to move file to the main array. 

Thankyou, I understand now.

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