Description

• take a Sun V40z server with 2 CPUs, 4GB of RAM
• add dual Fibre Channel cards for the local disk cache pool
• add more FC cards for tape(s)
• add dual-channel FC boxes as disk cache, two per tape
• connect up to two tapes

Reasoning

This is an implementation of the LocalCacheForTapeServers idea. The internal bandwidth of a PC can probably not handle the I/O needed, but a decent 64-bit box should. http://www.sun.com/servers/entry/v40z/arch-wp.pdf describes the internals of a v40z server and we found that there are no hardware bandwidth limitations in the way. In fact, two tapes could be served with one of these machines if needed.

Four disk pools would be used to cache incoming and outgoing streams. In this manner, a given pool would perform only writing or only reading operations at a time. Example:

• Disk A -> input from network
• Disk B -> idle
• Disk C -> input from tape
• Disk D -> streaming to the network

then, if eg. disk A fills up:

• Disk A -> writing to tape
• Disk B -> input from network
• Disk C -> idle
• Disk D -> streaming to the network

Then

• Disk A -> writing to tape
• Disk B -> input from network
• Disk C -> streaming to the network
• Disk D -> idle

Software

The box is proposed to run SLC3 or SLC4, with 2.6 kernels, using I/O prioritization (ionice) to ensure that tape(s) always get served first. Major changes are needed in Castor for the local disk pool management (someone please elaborate!). Unless FileMarks limitations in bandwidth are removed, we'll have no problem feeding the tapes anyway at 30MB/s...

-- AndrasHorvath - 02 May 2005