May 22, 201214 yr Hi, The title pretty much says it all. I'm very new to unraid and to linux as whole, but I just built my server got a plus license and have 6 drives (all 3TB 7200 RPM - 5x Seagate Barracuda; 1x Hitachi Deskstar) hooked and running and I'm 70% done copying all my files on to the array, all over a gigabit connection. I have been reading about the benefits of adding a cache drive to this existing setup, but can't seem to decide whether I should invest in one, my MoBo only has 6 SATA ports so this would mean getting a SATA controller PCIe card - like this one http://www.newegg.com/Product/Product.aspx?Item=N82E16816124045 and probably an SSD or a small HDD. So totally a little over $125 (with a 120GB SSD)! Any inputs to help me decide would be hugely appreciated - does this make transferring faster - what are the benefits on having one (if there are any)? Would an SSD be a better bet if I decide to add a cache drive? Thanks in advance for your help! - Abhi
May 23, 201214 yr A cache drive makes downloading/transferring faster, as it basically transfers to the cache drive, and then the cache drive is cleared once a day, moving all of the files to the array. Writing to the array directly is slower because it has to write to the disk and update parity at the same time, so with the cache drive it can update once a day, its set to shortly after 3 am I believe, when most people are asleep so it doesn't affect the user at all(unless your server is bare minimum specs you wouldn't notice anything anyway). A downside to this is any files you transfer during the day are not protected by parity until they are moved to the array. Many users can live with this downside as one day of transfers can be easily recovered, so the added transfer speed outways the risk. Pro's to doing this are faster transfer speeds, a "built in" place off of the array for storage, many users use the cache drive to store config data for plug-ins(if you do this it must be in a hidden folder so the mover doesn't attempt to move the folder). Also, if you use a big enough cache drive, the cache drive can act as a "warm spare", so if a drive in your array goes down, you can unassign the cache drive and re-assign it into the failed drives position, parity will rebuild and your protected again. If I had a choice, I'd choose the SSD for the speed boost. That is the main purpose of the cache drive, so going for as fast as possible seems logical. The size of the cache drive is wholly dependent upon how much data you expect to transfer on any given day. If you know you transfer less than 128GB a day, then that drive will work just fine. If you regularly transfer 300+ GB daily, it'd be wise to go for a larger capacity drive. Now, if you know you will transfer less than 128GB on most days, but occassionally exceed that, you can manually invoke the mover script from the webgui, and the 128GB SSD will still work fine. Only reason why I say if you transfer over 300GB a day go for a bigger drive, is you'd have to either 1. Make a cron to run the mover script more than once a day or 2. Invoke the script manually more than once a day, which could get annoying.
May 23, 201214 yr Honestly. I would think about getting another 3TB drive for your cache drive. Influencer has excellent points.. and yes I run an SSD cache drive.. BUT... 1 those seagate 3TB drives are pretty fast. while not SSD speed, they are quick. (dont forget that if your source is not SSD or raid you wont benefit from an SSD cache) 2 they hold a lot more then a 128gb SSD. therefore you can copy and copy and not worry about how much you copy.. 3 if one of your main drives fails, you can use the cache drive as an emergency spare to rebuild the array. 4 if your server runs out of room, you can assign the cache drive to the array while you wait for a sale on a new cache drive. @influencer.. I have my mover kick off every 2 hours.. with a sata3 SSD, you can write to it without speed loss while the mover is running.
May 23, 201214 yr Good point that unless the source is SSD/raid there is no benefit. Still stuck in the mindset that the PC I built for a friend had a SSD for OS and it was blazing fast, didn't immediately make the correlation of read vs write
May 23, 201214 yr Another "limitation" factor is the speed of your network. For most people this will be gigabit ethernet. A fast 'regular' drive is actually good enough to fill a gigabit link.
May 23, 201214 yr Author Thank you all. I am taking your advice and opting to go for a regular drive rather than an SSD. Since the drive I am copying from is an external HDD connected to the Windows machine via USB 3.0, over a gigabit internet, those would be the bottlenecks for transfer speeds and not the performance of the cache drive. So as you wise people said it makes sense to get a 7200 RPM 3TB warm spare. But the bigger question is whether I really do need one? Asking because Johnm said this: those seagate 3TB drives are pretty fast. while not SSD speed, they are quick. That being the case would I really see speed improvements when copying into the array vs cache drive? What are your thoughts on this? Thanks for the help so far.
May 24, 201214 yr Author Thank you BRiT - that is a great resource by Rajahal. Got most of my questions answered. This topic can now be closed or solved! Thanks for all the help.
May 24, 201214 yr This topic can now be closed or solved! Edit your first post, and change the title of the thread appropriately.
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