nanog mailing list archives
Return two locations or low TTL [was: DNS caches that support partitioning ?]
From: "Patrick W. Gilmore" <patrick () ianai net>
Date: Mon, 20 Aug 2012 06:24:20 -0400
While I hesitate to argue DNS with Mark, I feel this needs a response. On Aug 19, 2012, at 17:37 , Mark Andrews <marka () isc org> wrote:
In message <DDF607B5-415B-41E8-9222-EB549D3DBB0C () semihuman com>, Chris Woodfield writes:
What Patrick said. For large sites that offer services in multiple data = centers on multiple IPs that can individually fail at any time, 300 = seconds is actually a bit on the long end.
Which is why the DNS supports multiple address records. Clients don't have to wait a minutes to fallover to a second address. One doesn't have to point all the addresses returned to the closest data center. One can get sub-second fail over in clients as HE code shows.
I'm afraid I am not familiar with "HE code", so perhaps I am being silly here. But I do not think returning multiple A records for multiple datacenters is as useful as lowering the TTL. Just a few reasons off the top of my head: * How do you guarantee the user goes to the closer location if you respond with multiple addresses? Forcing users to go to farther away datacenters half the time is likely a poor trade-off for the occasional TTL problem when a DC goes down. * How many applications are even aware multiple addresses were returned? * How do you guarantee sub-second failover when most apps will wait longer than one second to see if an address responds? Etc. And that doesn't begin to touch thing such as cache efficiency that affect companies like Google, CDNs, etc.
As for the original problem. LRU replacement will keep "hot" items in the cache unless it is seriously undersized.
This was covered well by others. -- TTFN, patrick
Current thread:
- Re: DNS caches that support partitioning ?, (continued)
- Re: DNS caches that support partitioning ? Andrew Sullivan (Aug 17)
- Re: DNS caches that support partitioning ? valdis . kletnieks (Aug 17)
- Re: DNS caches that support partitioning ? Michael Thomas (Aug 17)
- Re: DNS caches that support partitioning ? Jimmy Hess (Aug 18)
- Re: DNS caches that support partitioning ? Patrick W. Gilmore (Aug 18)
- Re: DNS caches that support partitioning ? Chris Woodfield (Aug 19)
- Re: DNS caches that support partitioning ? Mark Andrews (Aug 19)
- Re: DNS caches that support partitioning ? William Herrin (Aug 19)
- Re: DNS caches that support partitioning ? Jimmy Hess (Aug 19)
- Re: DNS caches that support partitioning ? Gary Buhrmaster (Aug 19)
- Return two locations or low TTL [was: DNS caches that support partitioning ?] Patrick W. Gilmore (Aug 20)
- Re: Return two locations or low TTL [was: DNS caches that support partitioning ?] Dobbins, Roland (Aug 20)
- Re: Return two locations or low TTL [was: DNS caches that support partitioning ?] Patrick W. Gilmore (Aug 20)
- Re: Return two locations or low TTL [was: DNS caches that support partitioning ?] Dobbins, Roland (Aug 20)
- Re: Return two locations or low TTL [was: DNS caches that support partitioning ?] Patrick W. Gilmore (Aug 20)
- Re: DNS caches that support partitioning ? valdis . kletnieks (Aug 17)
- Re: Return two locations or low TTL [was: DNS caches that support partitioning ?] Mark Andrews (Aug 20)
- Re: DNS caches that support partitioning ? Andrew Sullivan (Aug 17)
- Re: Return two locations or low TTL [was: DNS caches that support partitioning ?] Tony Finch (Aug 20)
- Re: Return two locations or low TTL [was: DNS caches that support partitioning ?] Patrick W. Gilmore (Aug 20)
- Re: Return two locations or low TTL [was: DNS caches that support partitioning ?] Chris Adams (Aug 20)
- Re: Return two locations or low TTL [was: DNS caches that support partitioning ?] Patrick W. Gilmore (Aug 20)
- Re: Return two locations or low TTL [was: DNS caches that support partitioning ?] Tony Finch (Aug 20)