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I can't find a definitive description of how EBS handles write caching.

  • EC2
  • T2.medium instance type
  • EBS "gp2" devices
  • Linux (Ubuntu 14.04)
  • HVM virtualization

hdparm -I /dev/xvda doesn't work - presumably since EBS is a not a real block device.

Is there a succinct description of how to determine the write-cache settings on EBS?

JDS
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1 Answers1

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Per this: https://forums.aws.amazon.com/thread.jspa?threadID=29056

Someone from AWS commented this:

However, it is the case that when a write issued to a device backed by an EBS volume completes, that write is durably committed to the EBS volume. Failure of the instance at this point will not impact that data.

Unfortunately you have to log in to AWS to see the thread.

Good slides on performance and availability: https://forums.aws.amazon.com/thread.jspa?threadID=29056

Basically everything I've read says that when OS says the data is written, it written and reliable.

lsd
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  • Thanks for the repply. i've seen that thread; it is literally the only relevant discussion that comes up in a google search for this topic. (also, you don't actually have to log in to view it). Unfortunately, it doesn't quite answer my question. yes, EBS is "durable" once the block is written, but *when* is the block written? immediately of after caching? Or are you saying that write caching is "off" since when the OS determines the block is written, it actually is written. Hm, I reread the forum post and I think that's what you, and the forum thread, mean. Thanks – JDS Aug 31 '17 at 11:05
  • Yeah, that's what I think. Interesting though, it did make me log in. Weird. – lsd Aug 31 '17 at 12:37
  • So, totally off-topic: It asked me to log in initially; from experience, I knew that if I just revisited the forum site, or did so in a private browsing window, it would load without me logging in. :) – JDS Aug 31 '17 at 13:41