> On Jul 14, 2016, at 1:18 PM, Reindl Harald <[email protected]> wrote: > > > > Am 14.07.2016 um 20:54 schrieb Leif Hedstrom: >> >>> On Jul 14, 2016, at 12:11 PM, Randeep <[email protected]> wrote: >>> >>> Thanks Harald. >>> >>> I'll check the raid configuration. >> >> We recommend not using RAID for the ATS cache, it’s desirable to let ATS >> deal with that. I.e. just give it some JBOD’s. There can be some cases where >> you might want to RAID, but I personally do not. > > when you have a single cache disk which is overloaded with IOPS you have a > problem you can only solve with more disks > > without RAID in case of failed disks your storage space for cache goes down > and if you would not need the cache size why would you configure it at all? > > so *what* is the point against redundancy with improved performance?
Not to start a religious war here (but I probably just did), but there are plenty of reasons where you would not want to use RAID with ATS (or any caching proxy). For example (top of my head): * RAID 5/6 is generally slower than JBOD, and certainly reduces the available capacity * RAID 10 is very fast, at the expense of lost capacity (50%) * RAID 0 would be ill advised for ATS, because if you lose one drive, presumably you lose your entire stripe. And ATS has no way of dealing with that, so the entire cache is lost. * There’s likely overhead if you use software RAID, although I have no real numbers for this. I’d assume HW RAID is fine. ATS can manage devices on its own, removing failed disks, and continue running with those that remains. In general, I find this preferable; I rather run with e.g. 6 drives *most* of the time, and deal with reduced performance/capacity for the error case where I lose one drive (while ATS refetches). The alternative being (with RAID) to always run with reduced capacity (in the case of RAID 5/6 and RAID 10), or always reduced performance (RAID 5/6). That much said, I can see several very legitimate use cases and reasons to use RAID with ATS: * Ease of administration (as you point out, make the problem be someone else’s problem). * You might see better performance with e.g. RAID10, particularly on read’s, for the case where a large object is fetched very often from disk. This is because an ATS object never spans more than one physical drive, so RAID here can allow you to distribute that object across more than one physical disk. * Your data truly has to be resilient, such that losing a drive is seriously crippling. I dislike this, because a cache is a cache, if you depend on the content to always be there, it’s no longer a cache, it’s a file server. Cheers, — leif
