From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.223.130]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8836E800 for ; Fri, 22 Nov 2024 14:27:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=195.135.223.130 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1732285657; cv=none; b=YupffQjy7aXKolK08/PKKRqFGkJZDPzN1pDe/hEqDK0U+aLCuxM8Gfk+pMg81Y/mArEVk7hg2j+O1sapbvkLkYDDcAx7U7u5oA4wifKmgeKoDhZw+5+QX76yCDWmiIzxJVBHFN6PENC5A7kEZECfzr2a4dJX9zYNH/Iurx0g7ak= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1732285657; c=relaxed/simple; bh=COAzaN5TR5I7LJHEsZ/rWxg3xsa/mX59IZ2Q45H2Xr4=; h=Message-ID:Date:MIME-Version:Subject:To:References:From: In-Reply-To:Content-Type; b=Ooi0a00bEmjqCkSCFA/+FW6dsoGY7nvgcHxbjVI7FkKyRXKB4PeiJVTW+3k9z7lcJBD4CU0eahRGclU7JfDwi9tzADBZkimkgPL/c82naLzTA6BkD0CrRpOpPRE0ytZHbu/jQw+4vl5bPbwoOgRipXyn1HE1J8LoABimcKgje6k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=suse.de; spf=pass smtp.mailfrom=suse.de; dkim=pass (1024-bit key) header.d=suse.de header.i=@suse.de header.b=PdWzIWci; dkim=permerror (0-bit key) header.d=suse.de header.i=@suse.de header.b=wyxCZ/Qt; dkim=pass (1024-bit key) header.d=suse.de header.i=@suse.de header.b=PdWzIWci; dkim=permerror (0-bit key) header.d=suse.de header.i=@suse.de header.b=wyxCZ/Qt; arc=none smtp.client-ip=195.135.223.130 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=suse.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=suse.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=suse.de header.i=@suse.de header.b="PdWzIWci"; dkim=permerror (0-bit key) header.d=suse.de header.i=@suse.de header.b="wyxCZ/Qt"; dkim=pass (1024-bit key) header.d=suse.de header.i=@suse.de header.b="PdWzIWci"; dkim=permerror (0-bit key) header.d=suse.de header.i=@suse.de header.b="wyxCZ/Qt" Received: from imap1.dmz-prg2.suse.org (unknown [10.150.64.97]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 9FE2D21175; Fri, 22 Nov 2024 14:27:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1732285652; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Itw2NfPAmMgQLy8wO04E22cHyKNZfyjmOl13CVbYTk4=; b=PdWzIWciyanIloa20OTAc5gRi4NA0zQkPdX6zehDKs1xXe7fiF451IkOdWHLTmfTuW2p4N DoPTh/jbeqkVpUfxz1obvWbl6tj+PLF0qgENAU9jQa+LMkhGWb4Y2l6RvQAz+sPTuyi7TR bE5KGwS1HprZNctPS4loaqIHNYdpjXM= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1732285652; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Itw2NfPAmMgQLy8wO04E22cHyKNZfyjmOl13CVbYTk4=; b=wyxCZ/QtPOTB2g7SrDVSybtgL7TCb6vLEsRFwjtr3NU3ADvA6LwbhBZ+k9RSpbgV7nq0nu KXmkOkjknLQfSiBQ== Authentication-Results: smtp-out1.suse.de; none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1732285652; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Itw2NfPAmMgQLy8wO04E22cHyKNZfyjmOl13CVbYTk4=; b=PdWzIWciyanIloa20OTAc5gRi4NA0zQkPdX6zehDKs1xXe7fiF451IkOdWHLTmfTuW2p4N DoPTh/jbeqkVpUfxz1obvWbl6tj+PLF0qgENAU9jQa+LMkhGWb4Y2l6RvQAz+sPTuyi7TR bE5KGwS1HprZNctPS4loaqIHNYdpjXM= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1732285652; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Itw2NfPAmMgQLy8wO04E22cHyKNZfyjmOl13CVbYTk4=; b=wyxCZ/QtPOTB2g7SrDVSybtgL7TCb6vLEsRFwjtr3NU3ADvA6LwbhBZ+k9RSpbgV7nq0nu KXmkOkjknLQfSiBQ== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id 8EBDC138A7; Fri, 22 Nov 2024 14:27:32 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id j+fIIdSUQGdaaQAAD6G6ig (envelope-from ); Fri, 22 Nov 2024 14:27:32 +0000 Message-ID: <5353841e-2b6e-4ea0-9d46-08adb8925029@suse.de> Date: Fri, 22 Nov 2024 15:27:32 +0100 Precedence: bulk X-Mailing-List: linux-scsi@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC v1 0/8] scsi: Multipath support for scsi disk devices. To: himanshu.madhani@oracle.com, martin.petersen@oracle.com, linux-scsi@vger.kernel.org References: <20241109044529.992935-1-himanshu.madhani@oracle.com> Content-Language: en-US From: Hannes Reinecke In-Reply-To: <20241109044529.992935-1-himanshu.madhani@oracle.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Level: X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; RCVD_TLS_ALL(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_VIA_SMTP_AUTH(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FUZZY_BLOCKED(0.00)[rspamd.com]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_THREE(0.00)[3]; FROM_EQ_ENVFROM(0.00)[]; TO_DN_NONE(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:email,suse.de:mid,imap1.dmz-prg2.suse.org:helo] X-Spam-Score: -4.30 X-Spam-Flag: NO On 11/9/24 05:45, himanshu.madhani@oracle.com wrote: > From: Himanshu Madhani > > Hello Folks, > > Here is a very early RFC for multipath support in the scsi layer. This patch series > implements native multipath support for scsi disks devices. > > In this series, I am providing conceptual changes which still needs work. However, > I wanted to get this RFC out to get community feedback on the direction of changes. > > This RFC follows NVMe multipath implementation closely for SCSI multipath. Currently, > SCSI multipath only supports disk devices which advertises ALUA (Asymmetric Logical > Unit Access) capability in the Inquiry response data. > First of all, thank you for doing this. Had been on my to-do list for a long time. However, the one crucial thing why I kept pushing it back is: Residuals. NVMe native multipathing works because NVMe is a 'all-or-nothing' protocol, ie either the entire I/O had been completed, or nothing has happened. Which means for any failure we can safely retry the entire I/O on a different path (that's the 'steal_bio' thingie), knowing that it's safe to do so. For SCSI, however, this is not the case; it's perfectly valid for a target to do a partial completion, and ask the initiator to retry the remainders. And this partial completion might be at any position within the bvec, requiring us to resend the bio from a random starting position. Meaning we cannot do a blind 'steal_bio' thing. So: have you evaluated you series wrt to residuals? Have you _measured_ if residuals are happening? Have you considered your patchset how residuals could be treated? (It _might_ be possible to resend the entire I/O over to another path, even if the command had been partially completed. That's perfectly safe for reads, but for writes you have to be extremely careful to not cause a data corruption. We had some fun discussions here over at the NVMe side ...) And: please drop the device handler thingie for this, and concentrate on ALUA. No point in carrying legacy stuff around. _AND_ you have to evaluate the ALUA settings anyway to get a decent path selection. Cheers, Hannes -- Dr. Hannes Reinecke Kernel Storage Architect hare@suse.de +49 911 74053 688 SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich