On Sat, 26 Feb 2011 11:21:28 +0100 Piergiorgio Sartor <piergiorgio.sartor@xxxxxxxx> wrote: > On Fri, Feb 25, 2011 at 02:51:25PM +1100, NeilBrown wrote: > > On Thu, 24 Feb 2011 22:06:43 +0100 Piergiorgio Sartor > > <piergiorgio.sartor@xxxxxxxx> wrote: > > > > > Hi all, > > > > > > few posts ago was mentioned that the unit of the stripe > > > cache are "pages per device", usually 4K pages. > > > > > > Questions: > > > > > > 1) Does "device" means raid (md) device or component > > > device (HDD)? > > > > component device. > > In drivers/md/raid5.[ch] there is a 'struct stripe_head'. > > It holds one page per component device (ignoring spares). > > Several of these comprise the 'cache'. The 'size' of the cache is the number > > oof 'struct stripe_head' and associated pages that are allocated. > > > > > > > > > > 2) The max possible value seems to be 32768, which > > > means, in case of 4K page per md device, a max of > > > 128MiB of RAM. > > > Is this by design? Would it be possible to increase > > > up to whatever is available? > > > > 32768 is just an arbitrary number. It is there in raid5.c and is easy to > > change (for people comfortable with recompiling their kernels). > > Ah! I found it. Maybe, considering currently > available memory you should think about incresing > it to, for example, 128K or 512K. > > > I wanted an upper limit because setting it too high could easily cause your > > machine to run out of memory and become very sluggish - or worse. > > > > Ideally the cache should be automatically sized based on demand and memory > > size - with maybe just a tunable to select between "use a much memory as you > > need - within reason" verse "use a little memory as you can manage with". > > > > But that requires thought and design and code and .... it just never seemed > > like a priority. > > You're a bit contraddicting your philosopy of > "let's do the smart things in user space"... :-) > > IMHO, if really necessary, it could be enough to > have this "upper limit" avaialable in sysfs. > > Then user space can decide what to do with it. > > For example, at boot the amount of memory is checked > and the upper limit set. > I see a duplication here, maybe better just remove > the upper limit and let user space to deal with that. Maybe.... I still feel I want some sort of built-in protection... Maybe if I did all the allocations with "__GFP_WAIT" clear so that it would only allocate memory that is easily available. It wouldn't be a hard guarantee against running out, but it might help.. Maybe you could try removing the limit and see what actually happens when you set a ridiculously large size.?? NeilBrown -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
