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Robbie Ferguson

Do cache disks buffer part of a file, or only whole files?

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I have a cutesy little 8 GB SSD sitting in the package and never touched.  It's fast and solid, but useless due to its size.


Does unRAID need to cache the entire file(s) being sent over?  Or is it smart enough to know to move things at mid-way or when the cache drive is getting full.


For example, if I copy a 2GB file over and it caches, then a couple more 2 GB files, do I have to wait until that cache drive transfers the files 12 hours later?  Or does it know on the fly to do it so I can keep using the system?


What happens if I place a 25 GB file?  Does the cache drive burst bits of the data to the array, or does it just fail because my cache drive would be full and break everything?


Thanks  :)

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And it will not write halve files...


So basically no problem in using your smaller drive, you could increase the time interval for clearing it if you want, or you don't and it will still work (only write performance will be lower when it is full).


Personally I do have a cache drive but I do not really notice it, if I time the difference it is big but in real life I am either:


- Writing small files (documents and stuff), and those are written quickly enough without cache drive

- Very large media files (movies and stuff), and those are so large that I go and drink coffee anyhow..


Also make sure you do not copy from and to the array (so between shares) over your windows machine, it will work but is very slow, it is MUCH faster to telnet into the system and MC (midnight commander) your files around.. Enlarge the telnet window to your whole monitor for that good old DOS feeling :-)

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Typically, you have to set the min free space to a number larger than the largest file you will transfer. unRAID looks at the disk free space and the min free space setting to decide if the cache disk is good. If the free space is more than the min free space setting then it choses the cache disk and will try to write the file to it. So, you set 2gig as the min free and try to transfer that 25gig file. The empty cache has 8gig free which is > 2gig so unRAID will attempt to use the cache disk. Unfortunately, it will fail once it is filled as the file is being written.


Basically, you could set the min free just below 8gig and get 1 file transfer to the cache as long as your files are always <8gig.


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Thank you.

So yeah; at first glance, it seems "safe", but then I see your point: if the "minimum free" value is satisfied yet the file I'm transferring exceeds the free space of the disk, I'm in big trouble.


So I think what I'll do is scan my drives, see how big the largest file is, and if it's under 6GB, I'll set the minimum free to 6GB.  This way, I have a faster transfer when I transfer a couple 1GB files over (every Tuesday night, for example).  :)


THAT SAID: To those seeing this thread and wondering...


No, unRAID is by no means slow.  I have never felt that it is slow at all (unless the drives are just spinning up from sleep mode, but that's to be expected).  I edit HD video directly through my gigabit network to my unraid box WITHOUT a cache drive.


I'm only thinking of using a cache drive because I don't know what else to do with an 8 GB SSD  :)  I'd thought about using it as system RAM, but I already have 16 GB so I'm not lacking.


Thanks gang.




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I'm only thinking of using a cache drive because I don't know what else to do with an 8 GB SSD 

Do you have a table that has one leg shorter than the others?  Any door that you need to hold open just a little bit?  A bored pet that needs something new to chew on?


Just a few ideas...  :P


(OK - I'm going now...)

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Thank you so much S80_UK for reminding me so effectively that this was not the best $40 ever spent  ;)  haha!


Originally I was going to use it as a live encoding drive for 1GB show files, but the RAID 0 performs so well I've never needed the SSD.

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First off ...  Thanks so much for the original Unraid show, as it definately helped me with my first build.  As for a 8GB SSD drive, ... well.. things since the 1st show have progressed and in the age of 40TB common Unraid builds, that 8GB drive is equal or less than most thumb drives.  :)


A future show on the massive media server capabilities of a 20+ drive Unraid server would probably attract allot of viewers and spark allot of interest in the "geek world" which most of us live.  :D


Have to admire the fact you hate to part with that 8GB drive... I can TOTALLY relate!  lol




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