OT (slightly) swap limits
in [Debian]
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From: Damon L. Chesser on 24 Apr 2008 16:50 Being in an interview loop looking for employment, I find I am asked questions I never considered, for example: How much is enough swap? Simple, right? Try answering it. Used to be the limit was 2G files/partitions with up to 8 partitions total. This does not apply anymore: I have seen customers with 32G of swap. Not wanting to talk about what they did with 32G of swap, but I am trying to find documentation for the limits and the rules now. For example: 2xRAM (or 1.5) is not considered valid any more is it? If you have 64G of ram, you need 128G of swap. But in truth I don't know and I am not finding much on google (lots covering the 2G OLD limit). What I want is documentation so I am not working off of opinion (read religious belief). Got any links you want to share? -- Damon L. Chesser damon(a)damtek.com http://www.linkedin.com/in/dchesser -- To UNSUBSCRIBE, email to debian-user-REQUEST(a)lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmaster(a)lists.debian.org
From: Patrick Ouellette on 24 Apr 2008 17:10 On Thu, Apr 24, 2008 at 04:45:05PM -0400, Damon L. Chesser wrote: > > Being in an interview loop looking for employment, I find I am asked > questions I never considered, for example: How much is enough swap? > Rephrase the question. Ask what the intended use of the machine is, what response times the users expect, how much real RAM is there, and what applications/services will run off the machine. Then tell the interviewer how each parameter you've asked about would influence your decision on how much swap was enough. How much is enough? As much as the system needs to run and not kill processes due to lack of memory (real + swap). I've run machines with 1Gig or more RAM with NO SWAP. I've also run machines with 4Gig of RAM and 16Gig of swap (BIG datasets). Pat -- Patrick Ouellette pat(a)flying-gecko.net ne4po (at) arrl (dot) net Amateur Radio: NE4PO "Crank the amp to 11, this needs more cowbell - and a llama wouldn't hurt either" "Your arguments are an odd mix of overly optimistic on one side and overly pessimistic on the other" -- To UNSUBSCRIBE, email to debian-user-REQUEST(a)lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmaster(a)lists.debian.org
From: Damon L. Chesser on 24 Apr 2008 17:20 Patrick Ouellette wrote: > On Thu, Apr 24, 2008 at 04:45:05PM -0400, Damon L. Chesser wrote: > >> Being in an interview loop looking for employment, I find I am asked >> questions I never considered, for example: How much is enough swap? >> >> > > Rephrase the question. Ask what the intended use of the machine is, > what response times the users expect, how much real RAM is there, and > what applications/services will run off the machine. > > Then tell the interviewer how each parameter you've asked about would > influence your decision on how much swap was enough. > > How much is enough? As much as the system needs to run and not kill > processes due to lack of memory (real + swap). > > I've run machines with 1Gig or more RAM with NO SWAP. I've also run > machines with 4Gig of RAM and 16Gig of swap (BIG datasets). > > Pat > Well, I tend to agree with you, however, I am being sucked dry of my Linux knowledge (the purpose of the interview, find the point of breakdown to determine the extent of the knowledge/skill). And much to my surprise I just found this: "At a bare minimum, you need an appropriately-sized root partition, and a swap partition equal to twice the amount of RAM" page 59, from a questionable source: Installation Guide of RHEL5. and on page 62, they further detail it: "Swap should equal 2x physical RAM for up to 2 GB of physical RAM, and then an additional 1x physical RAM for any amount above 2 GB, but never less than 32 MB. So, if: M = Amount of RAM in GB, and S = Amount of swap in GB, then If M < 2 S = M *2 Else S = M + 2 Using this formula, a system with 2 GB of physical RAM would have 4 GB of swap, while one with 3 GB of physical RAM would have 5 GB of swap. Creating a large swap space partition can be especially helpful if you plan to upgrade your RAM at a later time. For systems with really large amounts of RAM (more than 32 GB) you can likely get away with a smaller swap partition (around 1x, or less, of physical RAM)." That is RHEL's take on the issue. Still looking for other sources. Interesting, I think. -- Damon L. Chesser damon(a)damtek.com http://www.linkedin.com/in/dchesser -- To UNSUBSCRIBE, email to debian-user-REQUEST(a)lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmaster(a)lists.debian.org
From: Damon L. Chesser on 24 Apr 2008 17:30 Damon L. Chesser wrote: > Being in an interview loop looking for employment, I find I am asked > questions I never considered, for example: How much is enough swap? > > Simple, right? Try answering it. Used to be the limit was 2G > files/partitions with up to 8 partitions total. This does not apply > anymore: I have seen customers with 32G of swap. > > Not wanting to talk about what they did with 32G of swap, but I am > trying to find documentation for the limits and the rules now. > > For example: 2xRAM (or 1.5) is not considered valid any more is it? > If you have 64G of ram, you need 128G of swap. But in truth I don't > know and I am not finding much on google (lots covering the 2G OLD > limit). What I want is documentation so I am not working off of > opinion (read religious belief). > > Got any links you want to share? > And because I think this is academically interesting I will post this link here as well: http://www.debian-administration.org/articles/550 interesting article on making a dynamic swap file at boot time that will be twice your RAM size automagicaly. -- Damon L. Chesser damon(a)damtek.com http://www.linkedin.com/in/dchesser -- To UNSUBSCRIBE, email to debian-user-REQUEST(a)lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmaster(a)lists.debian.org
From: Alex Samad on 24 Apr 2008 19:50 On Thu, Apr 24, 2008 at 05:16:56PM -0400, Damon L. Chesser wrote: > Patrick Ouellette wrote: >> On Thu, Apr 24, 2008 at 04:45:05PM -0400, Damon L. Chesser wrote: >> >>> Being in an interview loop looking for employment, I find I am asked >>> questions I never considered, for example: How much is enough swap? >>> >>> >> >> Rephrase the question. Ask what the intended use of the machine is, >> what response times the users expect, how much real RAM is there, and >> what applications/services will run off the machine. >> >> Then tell the interviewer how each parameter you've asked about would >> influence your decision on how much swap was enough. >> >> How much is enough? As much as the system needs to run and not kill >> processes due to lack of memory (real + swap). >> >> I've run machines with 1Gig or more RAM with NO SWAP. I've also run >> machines with 4Gig of RAM and 16Gig of swap (BIG datasets). >> >> Pat >> > Well, > > I tend to agree with you, however, I am being sucked dry of my Linux > knowledge (the purpose of the interview, find the point of breakdown to > determine the extent of the knowledge/skill). And much to my surprise I > just found this: > > "At a bare minimum, you need an appropriately-sized root partition, and > a swap partition equal to twice the amount of RAM" > > page 59, from a questionable source: Installation Guide of RHEL5. interestingly RHEL4 at 2.6 kernel distro still also made swap files of 2G max and used multiples of that. Since 2.6 this hasn't been necessary. Just because RHEL does it that way doesn't make it right Patrick I believe has the better approach - what are you going to use the box for and what sort of response do you want, although I have to diss agree on the need for large swap space for a database, DB are engineered to use all the space that are told to get hold of and have their own caching. Why interfere with it by pretending you have more memory than you do. You could end up hitting swap because the DB cache has grown. > > and on page 62, they further detail it: > > "Swap should equal 2x physical RAM for up to 2 GB of physical RAM, and > then an additional > 1x physical RAM for any amount above 2 GB, but never less than 32 MB. > So, if: > M = Amount of RAM in GB, and S = Amount of swap in GB, then > If M < 2 > S = M *2 > Else > S = M + 2 > Using this formula, a system with 2 GB of physical RAM would have 4 GB > of swap, while one > with 3 GB of physical RAM would have 5 GB of swap. Creating a large swap > space partition > can be especially helpful if you plan to upgrade your RAM at a later time. > For systems with really large amounts of RAM (more than 32 GB) you can > likely get away > with a smaller swap partition (around 1x, or less, of physical RAM)." > > That is RHEL's take on the issue. Still looking for other sources. > Interesting, I think. > > > -- > Damon L. Chesser > damon(a)damtek.com > http://www.linkedin.com/in/dchesser > > > -- > To UNSUBSCRIBE, email to debian-user-REQUEST(a)lists.debian.org with a > subject of "unsubscribe". Trouble? Contact listmaster(a)lists.debian.org > > -- "We want to develop defenses that are capable of defending ourselves and defenses capable of defending others." - George W. Bush 03/29/2001 Washington, DC White House press conference,
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