From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mimecast-mx02.redhat.com (mimecast01.extmail.prod.ext.rdu2.redhat.com [10.11.55.17]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 4743E2157F45 for ; Wed, 9 Sep 2020 19:44:37 +0000 (UTC) Received: from us-smtp-1.mimecast.com (us-smtp-1.mimecast.com [205.139.110.61]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-SHA384 (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id DEF208C0867 for ; Wed, 9 Sep 2020 19:44:36 +0000 (UTC) MIME-Version: 1.0 Date: Wed, 09 Sep 2020 21:44:22 +0200 From: Gionatan Danti In-Reply-To: <24409.9033.527504.36789@quad.stoffel.home> References: <79061390.1069833.1599071934227.JavaMail.zimbra@karlsbakk.net> <53661d4eefb635710b51cf9bfee894ef@assyoma.it> <83152674.4938205.1599663690759.JavaMail.zimbra@karlsbakk.net> <3503b4f5b55345beb24de4b156ee75c7@assyoma.it> <24409.9033.527504.36789@quad.stoffel.home> Message-ID: <7d62cb86425416d5a3db115afdbd996c@assyoma.it> Content-Transfer-Encoding: 7bit Subject: Re: [linux-lvm] Looking ahead - tiering with LVM? Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: LVM general discussion and development Cc: Roy Sigurd Karlsbakk , =?UTF-8?Q?H=C3=A5kon?= Il 2020-09-09 20:47 John Stoffel ha scritto: > This assumes you're tiering whole files, not at the per-block level > though, right? The tiered approach I developed and maintained in the past, yes. For any LVM-based tiering, we are speaking about block-level tiering (as LVM itself has no "files" concept). > Do you have numbers? I'm using DM_CACHE on my home NAS server box, > and it *does* seem to help, but only in certain cases. I've got a > 750gb home directory LV with an 80gb lv_cache writethrough cache > setup. So it's not great on write heavy loads, but it's good in read > heavy ones, such as kernel compiles where it does make a difference. Numbers for available space for tiering vs cache can vary based on your setup. However, storage tiers generally are at least 5-10X apart from each other (ie: 1 TB SSD for 10 TB HDD). Hence my gut fealing that tiering is not drastically better then lvm cache. But hey - I reserve the right to be totally wrong ;) > So it's not only the caching being per-file or per-block, but how the > actual cache is done? writeback is faster, but less reliable if you > crash. Writethrough is slower, but much more reliable. writeback cache surely is more prone to failure vs writethoug cache. The golden rule is that writeback cache should use a mirrored device (with device-level powerloss protected writeback cache if sync write speed is important). But this is somewhat ortogonal to the original question: block-level tiering itself increases the chances of data loss (ie: losing the SSD component will ruin the entire filesystem), so you should used mirrored (or parity) device for tiering also. Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8