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Avere bets filers are crying out for more tiers

Four storage levels starting with NVRAM

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Start-up Avere has designed a 4-tier filer appliance to deliver much faster I/O and dramatically lower the number of spindles needed in a high-performance filer.

With the Avere FTX Series, the company has undertaken a complete overhaul of filer architecture and moved on from the embedded server-plus-drive-trays approach characteristic of most network-attached storage (NAS) products. At the same time its product shares a few characteristics with F5/Acopia's ARX file virtualising resource switch. Specifically it sits in-line, in front of NAS arrays, interfacing them to accessing servers and presenting a single, global file namespace to those servers.

The filer sees Avere's FTX product as a server client and the servers see the FTX as a filer.

Ron Bianchini, Avere co-founder and CEO, says Avere is very conscious of the hard drive access density problem, the trend for hard drive capacity to rise faster than hard drive I/O performance. He says: "The performance per bit of hard drives is showing a 40 per cent decline a year." Or worse: "For 8KB blocks there's a 90 per cent reduction per year in random I/O performance per bit."

There's nothing existing NAS filers can do about this except suffer or over-provision and short-stroke. There's various other ways of trying to get out of the trap, such as clustering filers (Isilon), hardware-accelerating the processing (BlueArc) or caching the controller (NetApp).

The basic lesson seems to be that not all filer I/Os are equal and not all data should be read from or written to disk.

Avere's I/O profiling

What Avere's founders have done is look at the pattern of I/Os that hit filers and aimed to store data in the FTX in the most appropriate storage medium to provide optimum performance and cost per GB. There are three types of storage, apart from DRAM, in the FTX product: NVRAM (battery-backed non-volatile RAM) as a first tier, NAND flash solid state storage as a second tier, and 15,000rpm SAS hard disk drives in a third tier. The fourth tier in the hierarchy is SATA drives, located in a third-party filer and linked to the FTX by Ethernet.

The SATA drives are for long-term archival storage where SATA drives are "ten times cheaper than Fibre Channel which is ten times cheaper than single-level cell (SLC) NAND," according to Avere CEO and co-founder Ron Bianchini.

The I/O type is characterised by whether it's a read or write, sequential vs random, and its size. Avere reckons that bulk reads of large sequential files are best done from SAS hard drives. A DRAM buffer will front-end it with 2MB of file data at a time going into this and 4KB blocks streamed out of it the accessing server application.

Suiting storage media to I/O type

Small random reads are best done from DRAM which can sustain 300,000 4KB random IOPS. Bianchini compares this to SLC flash's 24,000 IOPS. The DRAM can be used to store file metadata and similar working data needing the fastest access.

SAS and NVRAM are favoured for writes. Random writes can go into the NVRAM, giving a good balance between speed and storage persistence. Where large file (sequential) write is needed it will be streamed into NVRAM in 4KB blocks and then 2MB at a time shipped out to the SAS drives. Bianchini says that this 2-step process is faster than writing to flash with its read-erase-write block cycle.

With a log file, again the random writes come in, are batched up in NVRAM and then written sequentially to a log file on hard drives.

Avere has three ways of decreasing disk drive latency: RAM hides access latency for sequential reads; NVRAM hides access latency for sequential writes; and log-based file systems minimize access latency for random writes.

Avere's software also characterises data by its activity level and type as well as by its I/O patterning. It automatically moves data (files and blocks within files) between the four tiers using access patterns, access frequency and data characteristics with no upfront or ongoing policy configuration, or waiting hours or days for promotion/demotion. The company calls this demand-driven storage.

Automated data movement is what EMC is promising with FAST but Avere has got there first for files, as Compellent got there before EMC for block data.

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