SSDs versus HDDs

2 HDD options  HDD options are well known  Enterprise/Desktop/Laptop  SGI sells virtually no desktop or laptop drives  15K, 10K, 7200 RPM  performance is well understood  behavior does not depend on previous workload  relatively constant over the life

3 SSD options  Flash type MLC/SLC/"enhanced" MLC  Over Provisioning percentage (OP)  Power Loss Protection (PLP)  Redundant Flash (SAFE/RAISE/XOR/RAID)  ECC  How many bits per sector?  How large an error can be corrected?  Warranty  Many SSDs (especially client or MLC) do not include write wearout in their warranty?  Those that do include wearout often use a Write Throttle

4 Client SSDs  Nearly all use MLC flash  Program/Erase cycles  Some as low as 1000 P/E cycles  Minimal OP to maximize user available GB  Typical around 7%, but exact amount difficult to determine  Many tech web sites have confusing data on OP, because 1.0 GiB == bytes  Some drive vendors do not tell how much raw flash they have  Most do not have PLP  Some have Redundant Flash  Most do not have a warranty for writes  Those that do use Write Throttling

5 Enterprise SSDs  SLC and MLC models  SLC rated for to P/E cycles  MLC rated for 5000 to P/E cycles  More OP to improve performance and wear life  Typically around 27%  Most have PLP  Most have Redundant Flash – If they do not have redundant flash, then they need high quality flash and excellent ECC  Most include writes in the warranty  Use write throttle with small P/E cycle ratings

6 DRAM SAS SSDs  Small capacity, very high performance  Power Loss Protection via writing contents of DRAM to flash  Good for filesystem logs, where multi-server access is important for HA, write duty cycle is high, and maximum performance is important.

7 PCIe Flash SSDs  Two types – SAS controller with SAS/SATA SSDs attached – Native PCIe FPGA/ASIC with Flash Memory  Observation: Native PCIe has lower latency than SAS/SATA SSDs

8 SSD performance  Highly dependent on – Flash controller and firmware design – Type of flash SLC versus MLC Asynchronous versus Synchronous (ONFI/Toggle) – Previous workload More for client than enterprise SSDs More for writes than for reads Less as OP is increased, either by vendor or by user via partitioning scheme that leaves part unused.

9 Performance Comparison (MB/s)

10 SSDs and XFS  XFS has “ibound” option, starting in ISSP2.2, and patched back to ISSP1.9  Allows building a concat volume with xvm or md, putting all inodes on SSD and data on HDD.  XFS log can go on SLC SSD or 15K HDD or DRAM SSD

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