Date: Wed, 07 May 2014 06:20:49 +0200 From: Rolf Nielsen <rmg1970swe@gmail.com> To: freebsd-questions@freebsd.org Subject: Re: Swap on ZFS Message-ID: <5369B4A1.40506@gmail.com> In-Reply-To: <C1DCD8EEE8523B91197B6D82@[192.168.1.50]> References: <CAHAXwYA9NBsYTpUGCZ_5f%2B_qDuvS%2BbN9vE4GAZhpRhWVWCwi7g@mail.gmail.com> <53697D8B.2060906@gmail.com> <C1DCD8EEE8523B91197B6D82@[192.168.1.50]>
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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 2014-05-07 02:56, Daniel Staal wrote: > --As of May 7, 2014 2:25:47 AM +0200, Rolf Nielsen is alleged to > have said: > >> On 2014-05-06 21:21, David Noel wrote: >>> Is swap on ZFS still ill-advised? All the forum, list, and >>> blog posts I find say it's a no-go. Is this still the case? The >>> idea behind it not working is that ZFS needs memory to write to >>> disk, so when you need to swap (are low on memory) ZFS won't be >>> able to write. >>> >>> I found some talk of having a tunable added as a workaround >>> that would reserve a certain amount of memory for ZFS so this >>> wouldn't be a problem, but have no idea if anyone's made any >>> progress towards implementing it. >> >> Just out of curiosity, why do you want it? >> >> To get swap on ZFS, you first need to create a ZFS filesystem on >> one or more devices, then you create a dedicated volume inside >> that filesystem and use that dedicated volume as swap. To me that >> seems to add unnecessary complexity, similar to using a file >> backed md device as swap. Please don't take this as criticism; >> you may very well have good reasons for wanting to do this. I'm >> just curious about those reasons. > > --As for the rest, it is mine. > > Because it's actually simpler than the alternative, in many cases. > The creating a ZFS filesystem is 'free' in this - you are only > planning on doing this if you are already running a ZFS-based > system, so you're already creating the filesystem. That leaves > creating the dedicated volume inside that and using it as swap - > which is as easy or easier than formatting and using a dedicated > disk as swap. > > So, really using swap on ZFS is no harder or easier than using a > dedicated swap disk, and no more complex. And if you *aren't* > planning on a dedicated swap disk, it starts adding complexity: If > you aren't using a dedicated swap disk, then you're probably > sharing it with a disk that you'll be using in the ZFS filesystem - > which means you now need to format and partition that disk, which > you didn't need to do before. You also have to monitor and > remember that the disk is partitioned, if you ever have to replace > it. (Which otherwise ZFS would make easy - just swap in a new one, > and tell ZFS to use it to replace the failed disk.) > > So your steps are: 1. Create ZFS filesystem. 2. Create swap inside > filesystem. 3. Configure FreeBSD to use swap. > > vs. 1. Partition Disks. 2. Set up Swap partition. 3. Configure > FreeBSD to use swap. 4. Create ZFS filesystem on other partition. > > Note of course that one of the points of using ZFS is the ease and > flexibility of creating volumes inside it - a ZFS user is probably > creating multiple at setup, and the swap volume isn't all that > different to create. And again, you're giving up the ability to use > ZFS to manage the device on the fly, which is one of ZFS's best > benefits. > > Daniel T. Staal > > --------------------------------------------------------------- > This email copyright the author. Unless otherwise noted, you are > expressly allowed to retransmit, quote, or otherwise use the > contents for non-commercial purposes. This copyright will expire 5 > years after the author's death, or in 30 years, whichever is > longer, unless such a period is in excess of local copyright law. > --------------------------------------------------------------- > _______________________________________________ > freebsd-questions@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-questions To > unsubscribe, send any mail to > "freebsd-questions-unsubscribe@freebsd.org" I'm not referring to the complexity of doing the setup, but to the added layer (a volume on a filesystem on a disk vs. a partition on a disk). I use ZFS for data storage, but I don't use any zvols. And my / is on UFS on a "dangerously dedicated" 40GB SSD that also has the swap partition. If I want to talk to my mother, I call my mother and talk to her. I don't call my sister and have her call my mother and relay everything. And for the same reason, I don't see why I should put a filesystem or swap on a volume on a filesystem. Rolf -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.22 (FreeBSD) iQIcBAEBAgAGBQJTabShAAoJEB1OKfQ0M8TguhEQALBrje+g/sTOCADfiFkOz4GO nEAfap3nYcq4ijakcdct5n3rAQQaH2rolCnL2YwYK2BQYqd7rRkdsQeE2A1DeyVI 09wbQsqzDK3JBlQfhP+HajDB0yasuE7DaRONxVYbeTVCkPbTbYES8+YPKvHTxAcK QPp18ParOrKCIpBYZDtIhq/L56f18lHzqvFz3m0kIUJxO3hx2y0MdwvGeojQUviU +pOlB3OJsaHLxoMFOC1EDpATfE9pORgTU6pqoN//JCSJPryPT2KaFIq2vT0FnTU7 pSvSsNf9dqGalyYrD0lkX1z80Q0EQ/xHr/G/+3/o16xgVNjX4ESOuMSeKnPrgbEt KPdJFHv1bHRRb1rJT0NZ4119Mdf8Rfk+VWUD1E14q08a6UFaJG/ji0KeLZZMK2EC dJa4AJH7QVpy0NalofxOl1Q6R9x8IoXCMbWTyRMubVQzefJGvN7UdoC9HpBy+8U8 6UiTkSOIJENgXrrO85ytB+rzRN7yS/SCYV0+UiMmc2u+AXmguUOKd2dwjVJy0K58 E0tsBjXYmNpnekkpJGtygxkBrs1jftEp38rWPjHKo/0CGwvhlz+cuNe1xrMI/tIK f1qOotlPKuPXUp/RosrB1+bTxQD6kA6rRR7AeWYKMRyGrGNWGrNltCeRnJvkqIxs f4hb8quzSUYc3JVq/8Lu =6lhO -----END PGP SIGNATURE-----
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