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Chapter 11. Central Flushing to tertiary storage systems

Patrick Fuhrmann

This chapter is of interest for dCache instances connected to a tertiary storage system or making use of the mass storage interface for any other reason.


The central flush control is still in the evaluation phase. The configuration description within this chapter is mainly for the dCache team to get it running on their test systems. The final prodution version will have most of this stuff already be configured.

dCache instances, connected to tertiary storage systems, collect incoming data, sort it by storage class and flush it as soon as certain thresholds are reached. All this is done autonomously by each individual write pool. Consequently those flush operations are coordinated on the level of a pool but not globally wrt a set of write pools or even to the whole dCache instance. Experiences during the last years show, that for various purposes a global flush management would be desirable.

Separation of read/write operations on write pools

The total thoughput of various disk storage systems tend to drop significantly if extensive read and write operations have to be performed in parallel on datasets exceeding the filesystem caches. To overcome this technical obstacle, it would be good if disk storage systems would either allow writing into a pool or flushing data out of a pool into the HSM system, but never both at the same time.

Overcoming HSM limitations and restictions

Some HSM systems, mainly those not coming with their own scheduler, apply certain restrictions on the number of requests being accepted simultaniously. For those, a central flush control system would allow for limiting the number of requests or the number of storage classes being flushed at the same time.