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Thread: [SOLVED] If my server has 4GB memory, how many users can support?

  1. #11
    jholder's Avatar
    jholder is offline Former Zimbran
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    BTW, this is the Zimbra Desktop Client forum I'm gonna move this to the administrators Zimbra Server forum.

    Let us know if you need anything else!

  2. #12
    quietas is offline Elite Member
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    I see it's solved, but I'll throw in my two cents real quick. I have 220 users with an HP DL380 using dual 3.06 Xeons, 4x Sata II 320gb drives (7200rpm), and 4gb RAM. As for software this is 5.0.8 NE on Ubuntu 6.06.2 32bit. So far it's cooking along with no issues or complaints.

    Here is my top info for reference with 70 users logged in now. 45 web, 1 admin, 24 via IMAP without Outlook connector.
    top - 13:14:09 up 16:52, 1 user, load average: 0.51, 0.62, 0.51
    Tasks: 163 total, 2 running, 161 sleeping, 0 stopped, 0 zombie
    Cpu(s): 0.7% us, 0.1% sy, 0.0% ni, 99.3% id, 0.0% wa, 0.0% hi, 0.0% si
    Mem: 3831692k total, 3676528k used, 155164k free, 86136k buffers
    Swap: 11221360k total, 96k used, 11221264k free, 1294740k cached
    It is definately RAM hungry and if you can do 64bit with 8gb RAM, I'd recommend it. RAM is cheap, plan for overkill if you can. This server I am using now will later be the failover and I am planning a 16gb Beast with RHEL 64 and 6 SAS drives for IO performance.
    Culley
    Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
    Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now

  3. #13
    dspillett is offline Loyal Member
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    Quote Originally Posted by jholder View Post
    The real question is 64-bit or 32. You said 32 bit. It's a big misconception that 4GB on 32 bit=4 GB on 64 bit. That's not true. It's more like 4GB on 32 bit = 2 GB on 64 bit?
    Why? The bits are larger, therefore they require more memory
    I'm trying to work out whether your are joking or really believe that. In case you are not joking...

    4GB on 32 bit is nothing like 2Gb on 64 bit. Yes, pointers are going to be larger (64 bits, not 32) which will make the index elements of data structures larger. Yes, some structures will be larger due to alignment issues (it is sometimes more efficient to 64-bit align data, which in some structures will leave extra gaps). Yes, some code is larger due to some larger instructions, some larger stored values, and alignment-for-efficiency issues.

    But a 10Kb email is still going to be 10Kb of data in RAM, same for a 1Mb email, or 1Mb worth of data from the database in RAM. The data might be pointed to by a reference value that is twice as large as on a 32 bit system and might be "wasting" some extra bytes due to 64-bit alignment, but the data itself will not be any bigger and as it is the majority of what is in RAM (code and indexing structures for data structures being a small part of what is in RAM) moving to a 64-bit architecture will not effectively half the benefit of your RAM.

    It might add 1%. Maybe 2%. Maybe a little more. But you'll lose far more of your 4Gb RAM in a 32 bit system due to a large part of the last Gb being wasted by limitations of the 32 bit address space and the fact that hardware devices need to be mapped somewhere (I have a not-too-old HP rack mounted server here where 32-bit Windows (I've not tried Linux on that box) will only see 3.25Gb of the 4 it has wasting 768Mb, people report 512Mb being the most common amount of 4Gb "not seen" with some setups "wasting" only 256Mb, and the *best* I've seen personally is 32-bit Linux on my home server which can use all but just over 200Mb of the 4Gb).

    Looking at quietas' example, his machine's current RAM use is:
    3741Mb total usable, of which:
    3590Mb is in use and
    151Mb is currently unused
    Of the 3590 that is in use 1348 is being used for buffer/cache, and no swap is in use, so the machine seems to have enough RAM for current load, but would probably benefit from more (for use as extra cache/buffer) and would certainly suffer noticeably with less.

    So go 64-bit, even with 4Gb RAM or less. Saves messing around rebuilding/upgrading later if/when you need to grow the RAM beyond 4Gb (or 3-point-something as a 32-bit OS won't use the whole 4Gb).

    Another small note: you can actually use more than 4Gb of RAM on 32-bit arrangements by using PAE. But this can harm performance measurably, and has other limitations, so is generally not something you would chose to do instead of installing a 64-bit OS.
    Last edited by dspillett; 07-31-2008 at 05:51 AM.

  4. #14
    quietas is offline Elite Member
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    The whole concept of 64bit systems using 4gb as though it were 2gb is completely wrong. 64bit uses 4gb as though it were 4gb, whereas 32bit uses 4gb as though it were 3.5gb. The Old New Thing : Why can't I see all of the 4GB of RAM in my machine? has a good explanation, but 32bit will only address 3.5gb roughly due to some being held back for hardwhare and ROM usage. Using PAE can free it up, but 64bit is easy. My laptop boots 64bit Vista and 64bit Ubuntu, both see 4096, XP on he same only saw 3454 or something about there.

    When my machine was being hammered swap did get used a bit yesterday, not much though. I am using 32bit Ubuntu 6.06 server so that limits it a bit. With RAM prices and nearly all new chips being 64bit, just get 8gb with a 64bit OS and save yourself some headaches.
    Culley
    Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
    Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now

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