Mailinglist Archive: opensuse (1470 mails)

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Re: [opensuse] experiences with bache / logic of caching
  • From: Per Jessen <per@xxxxxxxxxxxx>
  • Date: Wed, 17 Feb 2016 12:28:18 +0100
  • Message-id: <na1lgj$o0m$1@saturn.local.net>
Greg Freemyer wrote:

Adding RAM is often the cheapest way to improve performance with a *NIX
system that can make use of it. Again there are a few "if" in that.
This may not be your bottleneck. Profiling is important. You may also
come up against hardware limits. Many PC motherboards have too few
memory slots or their chip set won't allow the use of larger capacity
memory cards. But before you spend >$50 on a SSD
http://www.amazon.com/SanDisk-6-0GB-2-5-Inch-Height-SDSSDP-064G-G25/dp/B007ZWLRSU/ref=sr_1_3?ie=UTF8&qid=1455464006&sr=8-3&keywords=64g+ssd
consider spending the same on RAM

A worthy comment, but RAM is still $7/GB or so and high-speed SSD is
down below $0.50/GB

I'll have to check, but I don't think adding memory will do much. The
filesystems cache is not used because there are no filesystems in use - the
raw disk space is handed out over iSCSI, just as Greg described.

I've bought at least 4TB of SSD in the last 6 months. At $7/GB that
would be a $28K expense vs the $2K expense it was.

What I'm looking for is an economical compromise that will keep this storage
setup efficient for maybe another 2-3 years. A PCIe SSD card with e.g.
400Gb is only about CHF500.


There are low-level ATA/SCSI commands that you seem to ignore. If an
app wants to control the caching scheme it can use O_DIRECT open()
flag to bypass the kernel caching and implement its own.

I'd be surprised if there isn't a way for userspace to trigger FUA /
DPO / FLUSH_CACHE commands.

Probably fcntl() and ioctl().


/Per

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