Product : DataCore, SSY [SDS]/10.0 PSP12, x86
Feature : Memory Purpose, Memory, Storage Support
Content Owner:  Herman Rutten
Read/Write Cache
DataCore SANsymphony accelerates reads and writes by leveraging the powerful processors and large DRAM memory inside current generation x86-64bit servers on which it runs. Up to 8 Terabytes of cache memory may be configured on each DataCore node, enabling it to perform at solid state disk speeds without the expense. SANsymphony uses a common cache pool to store reads and writes in.

SANsymphony read caching essentially recognizes I/O patterns to anticipate which blocks to read next into RAM from the physical back-end disks. That way the next request can be served from memory.

When hosts write to a virtual disk, the data first goes into DRAM memory and is later destaged to disk, often grouped with other writes to minimize delays when storing the data to the persistent disk layer. Written data stays in cache for re-reads.

The cache is cleaned on a first-in-first-out (FiFo) basis. Segment overwrites are performed on the oldest data first for both read- and write cache segment requests.

SANsymphony prevents the write cache data from flooding the entire cache. In case the write data amount runs above a certain percentage watermark of the entire cache amount, then the write cache will temporarily be switched to write-through mode in order to regain balance. This is performed fully automatically and is self-adjusting, per virtual disk as well as on a global level.