Barton Robinson wrote: > I think I have a different perspective. > > Storage is a valueable resource. Caching takes storage. You > have a choice of where to cache, your xx number of linux > servers could all be big enough to cache. That is inherantly > bad. Sizing of linux virtual machines is meant to reduce the > cache size because linux will always use it's storage for cache, > it could have data cached that has not been referenced for > hours or days - not a good use of our precious storage. > > Another part of this consideration, with xx servers, they > are not all likely to be active at one time. So we need a > way to move the cache storage from the idle servers to the > active ones. If linux owns the cache, this is not possible. > We control the linux server cache size > by minimizing the linux servers, so that there is less room for > cache. > > Our z/VM technology works because it shares resources. > sharing storage is a big benefit. so how to share the cache? > use MDC. > With MDC, there is one cache that by nature will cache > the most current data for the currently active servers. > So what if this is one disk only used by one server. It > is the active server that needs the cache, not the inactive > ones. > > Then use a "good performance tool, esalps comes to mind" > to determine if there are some disks or servers that receive > no benefit and disable those disks or servers for mdc. > ESALPS does report cache hits by disk, and by user. so there > is LOTS of tuning feedback available to those of you with > decent performance tools. > > > There is a lot of research that has not been done on MDC > in linux on vm environment. > like none. The impact of record level caching vs track > caching i believe improves the cache effectiveness by > factor of 10 for simple reason that linux references > one block on a track and MDC caches the whole track. > Rarely does linux actually use multiple blocks on a > single track so MDC wastes channel time, disk time > and MDC storage - clearly wasting resource. > But, no body uses record level caching, even > though it is signficantly better for linux workloads. > > And there is no reason to believe that allowing disks > that are only used by one server to be mdc cached is a > bad idea. How else can you give a server a couple hundred > MB of cache dynamically when it needs it? I second your thoughts regarding block vs track caching, but I doubt that a scenario exists where MDC for non-shared mdisks outperforms reasonable distribution of the available storage to the linux images. Would you care to show such measurement? --
Carsten Otte IBM Linux technology center ARCH=s390 ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
