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Today's Topics:
1. Re: [EXTERNAL] Re: extremely slow DHCPD startup time
(Shane Merritt)
2. Re: extremely slow DHCPD startup time (Timothe Litt)
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Message: 1
Date: Thu, 4 Aug 2022 20:18:04 +0000
From: Shane Merritt <[email protected]>
To: Users of ISC DHCP <[email protected]>
Subject: Re: [EXTERNAL] Re: extremely slow DHCPD startup time
Message-ID:
<ds0pr15mb5400c1317b6138e3aca377d8a6...@ds0pr15mb5400.namprd15.prod.outlook.com>
Content-Type: text/plain; charset="windows-1252"
The server disks are already SSD. The total max available lease space
according to dhcpd-pools is 283,751. I did a restart on one of the pair
earlier and the time was about 10 minutes, but activity is low at the moment
since fall semester hasn?t begun.
Shane Merritt
Office of Information Technology
The University of Alabama
[email protected]
From: dhcp-users <[email protected]> on behalf of Sten Carlsen
<[email protected]>
Date: Thursday, August 4, 2022 at 2:54 PM
To: Users of ISC DHCP <[email protected]>
Subject: [EXTERNAL] Re: extremely slow DHCPD startup time
On 4 Aug 2022, at 21.05, Greg Sloop <[email protected]<mailto:[email protected]>>
<[email protected]<mailto:[email protected]>> wrote:
I'm sure others will have some useful suggestions - but I do know that
improving disk I/O can seriously improve performance.
I'm not sure if that will improve restart/start/reload times.
It would be pretty easy to test though, say swapping for an SSD/Nvme.
(Or if you like living dangerously, putting the disk in RAM.)
On Thu, Aug 4, 2022 at 12:02 PM Shane Merritt
<[email protected]<mailto:[email protected]>> wrote:
I have a failover pair of DHCP servers running 4.4.2-P1 that handle a DHCP for
a very large wireless network (>75k devices active at any time). The leases
file on these boxes is around 200 megabytes. The problem I am having is when I
need to make a change and restart, DHCPD takes over 25 minutes to start giving
out addresses. I assume this is due to the large lease file. I see that there
are tons of leases in the file that are ?binding state free? that in some cases
are months old. Are there any suggestions to improve this situation?
AFAIK the server will fill its internal tables with every possible lease, most
of them empty or marked free. I could be wrong about this.
So the question is how large your net blocks are?
If you e.g. have a /8 address space, that would amount to the server having to
populate 2^24 (16 mill) entries in its internal table and do what ever
processing etc it does for each entry. This may take time.
Shane Merritt
Office of Information Technology
The University of Alabama
[email protected]<mailto:[email protected]>
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Message: 2
Date: Thu, 4 Aug 2022 19:54:38 -0400
From: Timothe Litt <[email protected]>
To: [email protected]
Subject: Re: extremely slow DHCPD startup time
Message-ID: <[email protected]>
Content-Type: text/plain; charset="utf-8"; Format="flowed"
On 04-Aug-22 15:51, [email protected] wrote:
> Date: Thu, 4 Aug 2022 12:05:46 -0700
> From: "Greg Sloop<[email protected]>"<[email protected]>
> To: Users of ISC DHCP<[email protected]>
> Subject: Re: extremely slow DHCPD startup time
> Message-ID:
> <CAAjorqX9EnKdwW=3go3vycx_tvqthf4je1ayrrdc+8boxye...@mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> I'm sure others will have some useful suggestions - but I do know that
> improving disk I/O can seriously improve performance.
> I'm not sure if that will improve restart/start/reload times.
>
> It would be pretty easy to test though, say swapping for an SSD/Nvme.
> (Or if you like living dangerously, putting the disk in RAM.)
To live less dangerously, create a partition for your lease file on a
real disk.? lvm or mdadm mirror (RAID 1) it with a RAM disk, and set the
real disk to -writemostly (lvm) / -W (mdadm.
Writes will be limited to the speed of the real disk, but reads (thus
startup) will go at RAM disk speeds.
Startup/shutdown needs to create/remove the RAM disk.
https://www.howtoforge.com/hybrid-raid_1-mirror-of-ram-drive-and-sata-hdd-using-lvm-with-luks-and-systemd-on-fedora-linux
has a basic example (near the end).
For system boot, you can make dhcpd start after the RAM disk is
initialized and mirror copy is complete or you can have the initial
dhcpd startup run from the real disk and let the mirror catch up.? The
former is probably faster (the real disk will be read only by the mirror
copy), but you should benchmark it since YMMV.
tmpfs can be used instead of the RAM disk.
You should also make sure that dhcpd's ulimits (and system memory)
suffice to avoid pagefaults/swapping while dhcpd is building its
in-memory structures from the leases file.
Don't forget to allocate enough space for both leases and leases~ in the
partition & RAMdisk.
Timothe Litt
ACM Distinguished Engineer
--------------------------
This communication may not represent the ACM or my employer's views,
if any, on the matters discussed.
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